{'fraction_non_alphanumeric': 0.02495201535508637, 'fraction_numerical': 0.02326670099714433, 'mean_word_length': 5.602225312934632, 'pattern_counts': {'":': 0, '<': 1, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 1, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 1, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Suicidality in sleep disorders: prevalence, impact, and management strategies

Sleep disturbances are associated with suicide-related thoughts and behaviors, and the incidence of sleep concerns and suicide has increased recently in the US. Most published research exploring the sleep–suicidality relation is focused on select sleep disorders, with few reviews offering a comprehensive overview of the sleep–suicidality literature. This narrative review broadly investigates the growing research literature on sleep disorders and suicidality, noting the prevalence of suicide ideation and nonfatal and fatal suicide attempts, the impact of several sleep disorders on suicide risk, and potential sleep-disorder management strategies for mitigating suicide risk. Aside from insomnia symptoms and nightmares, there exist opportunities to learn more about suicide risk across many sleep conditions, including whether sleep disorders are associated with suicide risk independently of other psychiatric conditions or symptoms. Generally, there is a lack of randomized controlled trials examining the modification of suicide risk via evidence-based sleep interventions for individuals with sleep disorders.


Introduction
Globally, approximately 788,000 individuals died by suicide in 2015, 1 and it is estimated that one suicide occurs every 40 seconds somewhere in the world. 2 In the US, the crude suicide rate increased by 19% from 1999 to 2010 (10.5-12.4 per 100,000 individuals). 3 During this same period, there was a 226% increase in outpatient visits related to sleep disorders among adults aged 20 years and older, with the number of sleep-medication prescriptions increasing by 293%. 4 Concerns about sleep problems in other countries have also been noted, with a study of eight countries showing that more than 20% of sampled adults reported poor sleep quality in Bangladesh, South Africa, and Vietnam. 5 Sleep disturbances have been identified as a risk factor for suicide ideation, nonfatal suicide attempts, and suicide, [6][7][8] and specific sleep-disorder symptoms (ie, insomnia and nightmares) have demonstrated unique relations to suicide-related thoughts and behaviors. 9 Given the aforementioned concerns regarding the prevalence of suicide and sleep problems, including recent increases in the US suicide rate and outpatient visits for sleep-related complaints, we briefly reviewed the research literature on suiciderelated thoughts and behaviors in the varying sleep disorders to provide readers with an updated overview of the current literature. Additionally, we aimed to provide one of the first comprehensive reviews of the growing literature on suicide risk and sleep disorders, since past reviews of sleep disorders and suicidality have focused on a circumscribed number of sleep conditions. 6,7,9,10 Finally, this review highlights the available evidence on the prevalence of suicide risk among sleep disorders, the impact of sleep disorders on suicide risk, and potential sleep-disorder management strategies for mitigating suicide risk.
For this narrative review, studies were included if 1) any measure of sleep disturbance and suicide risk was administered in the study, 2) results were published, in press, or currently being considered for publication (ie, under review) in a peer-reviewed journal, and 3) the paper or published conference abstract was written in or translated into the English language. Our search strategy for identifying relevant studies included a search of all EBSCOhost databases, including manual electronic searches of Medline (no start date to May 2017), PsycInfo (no start date to May 2017), and Google Scholar (no start date to May 2017). Examples of general search terms used to identify relevant published work included "sleep", "sleep disorder", "sleep disturbance", "suicide", "suicidal", "suicide risk", "and suicidality". In addition to the general sleep terms utilized, the first author individually paired the names of the seven major categories of sleep disorders (eg, parasomnias) from the third edition of the International Classification of Sleep Disorders (ICSD) 11 with each of the suicide-related search terms noted in the previous sentence. The first author also did this for every sleep-disorder diagnosis (eg, nightmare disorder) listed in the seven major categories to ensure that information was included for all of the sleep-disorder diagnoses listed in the most recent edition of the ICSD.

Insomnia symptoms
Along with the increased prevalence of outpatient visits noted previously, outpatient visits due to insomnia symptoms have increased 13% from 1999 to 2010 in the US. 4 Epidemiological studies estimate that up to 30% of the general population may have experienced insomnia symptoms during the past month. 12,13 Prevalence estimates of adults receiving an insomnia diagnosis is substantially lower than prevalence estimates of insomnia symptoms, [13][14][15] though they have also increased (ie, from 11.9% to 15.5% over a 10-year period). 16

Prevalence of suicidality in insomnia
There is a paucity of research estimating the prevalence of suicide-related thoughts and behaviors among individuals with insomnia. Studies examining insomnia and suiciderelated variables often do so in samples where a diagnosis of clinical insomnia is not a focal point of sample recruitment. Several studies, though, have provided some perspective on the incidence of insomnia symptoms in samples with at-risk participants (see Table 1). For participants with insomnia symptoms, the prevalence for suicide ideation was 25%-32%, 17,18 7%-92% for nonfatal attempts, 17,[19][20][21][22] and 11%-37% among suicide decedents, with higher percentages appearing more closely to the time of death. [23][24][25][26] 69 The odds of suicide were fivefold greater in a sample of adolescents experiencing insomnia symptoms within the week preceding suicide, with suicide decedents more likely than controls to experience a worsening of sleep disturbance in the final week preceding suicide. 23 Additionally, among a sample of veteran suicide decedents, those decedents with documented sleep disturbance (ie, insomnia or hypersomnia) died significantly sooner than decedents without documented sleep disturbances. 64 Longer time frames to suicide have also been observed, as a dated 10-year prospective study of 954 psychiatric inpatients with a major mood disorder showed that global insomnia (presence of onset, maintenance, and terminal insomnia) was one of six variables associated with a fatal suicide attempt within a 1-year time frame. 63 Taken together, these results suggest that insomnia may function as a distal or proximal risk factor for suicide, with a role in both imminent and long-term suicide risk.

Insomnia and suicidality inconsistencies
Diverging from the studies reviewed thus far, Pompili et al 70 found no significant differences in suicide ideation or suicide-attempt history between those reporting and denying insomnia symptoms in a sample of 843 patients admitted to an emergency department in Rome. However, they did find that those reporting insomnia symptoms more frequently used a violent suicide-attempt method. Furthermore, a few studies have shown that the presence of a psychiatric illness 24 or depressive symptoms 36,41,52,71 have an indirect effect on the relation between insomnia symptoms and suicide-related variables. However, across numerous studies, associations between insomnia symptoms and suicide-related variables have been independent of depressive symptoms and/or other potential confounders (ie, age, sex, fatigue, chronic health problems, anhedonia, hopelessness, anxiety symptoms, substance use, cognitive ability, or the presence of any psychiatric illness). 19,23,25,30,31,33,34,40,42,46,48,51,57,61,64,66,68,69,[72][73][74] Additional inconsistencies include the risk conferred by specific insomnia symptoms (see Table 2). Research does not show a clear pattern regarding whether onset, maintenance, or terminal insomnia independently heighten suicide risk, with several studies showing that concurrently reporting two of these three insomnia symptoms may confer greater risk for suicide ideation 40,42,46,57 and attempts, 30,31,75,76 intent to attempt suicide, 72 and death by suicide. 66 This suggests that greater risk may result from the presence of multiple sleep disturbances. 54,66,73 In an attempt to understand the insomnia-suicide relation better and perhaps account for the aforementioned Table 2 Inconsistencies in insomnia symptoms and suicidality research

Initial insomnia
Initial insomnia has been associated with current 51 and past 12-month suicide ideation, suicide-attempt planning, and nonfatal suicide attempts, 73 and has been independently associated with youth suicide, 23,24 but was shown to be protective against attempting suicide at age 15-20 years. 193

Middle insomnia
Middle insomnia has been independently associated with suicide in youth 24 and adult 65 samples, and has been associated with an increased frequency of lifetime suicide attempts, 140 with adolescents reporting middle insomnia showing a four- 179 to sixfold 193 greater likelihood of reporting a past suicide attempt than those without middle insomnia. The relation between middle insomnia and suicide ideation, however, has not been consistent, though it should be noted that the majority of studies have shown an association between these two variables and the few inconsistencies do not appear to depend on whether researchers adjusted for psychiatric illness or other covariates, given that this was done across the majority of studies. 30,31,40,42,46,57,140

Terminal insomnia
Regarding terminal insomnia, a few studies have shown that terminal insomnia is associated with suicide ideation (independently of mood, anxiety, and substance-use disorders), 140 greater intent to attempt suicide, 128 and recent suicide attempts, 73,76 but not lifetime suicide attempts. 140 A study of Chinese adolescents showed that terminal insomnia did not predict significantly greater odds of suicide ideation when adjusting for age, sex, father's occupation, and depressive symptoms. 117 Nature and Science of Sleep 2017:9 submit your manuscript | www.dovepress.com

216
Drapeau and Nadorff inconsistencies, researchers have explored several potential mechanisms 77 (see Table 3 for a summary). It is important to note that much of the mediation research thus far has been atemporal, due to the cross-sectional or baseline-follow-up research methods employed, limiting conclusions regarding temporality. 78 More than two time points may be necessary, in order to infer temporal ordering of mediation. 78

Management of suicidality and insomnia
Researchers have continuously called for greater empirical focus on the investigation of insomnia-treatment effects on mitigating suicide risk. 33,64,[79][80][81][82][83] Although studies examining the efficacy of insomnia treatments on reducing suicide risk have been conducted, 84 there is a significant need for more research (ie, randomized controlled trials). Cognitive Table 3 Summary of mechanisms examined in the insomnia-suicidality relation

Proposed mediator Summary of research Hopelessness
Hopelessness had an indirect effect on the insomnia-suicide ideation relation in a cross-sectional sample of US adults, independently of depressive symptoms. 194 However, other studies have shown that the insomnia symptom-suicide ideation relation remains when adjusting for depressive symptoms and feelings of hopelessness. 48 Additionally, a case study of a 64-year-old male admitted to a psychiatric hospital after a suicide attempt noted that the patient's feelings of hopelessness were secondary to insomnia distress and resolved, along with suicide ideation, following sleep improvements. 195 Finally, in a study of adults with depression, hopelessness did not relate to insomnia symptoms or dysfunctional beliefs and attitudes about sleep, 43 which is notable, given that it has been argued that some of the items on the DBAS measure reflect feelings of hopelessness. 196

Depression structure
Allan et al 71 suggested that the structure of depression (ie, cognitive/affective symptoms vs somatic) may explain the discrepant depression-mediation results across studies. In their sample of 405 current and former US military personnel, cognitive/affective symptoms of depression had an indirect effect on the relation between insomnia symptoms and baseline and 12-month follow-up suicide-ideation levels.

Insomnia duration
Research has shown that the duration of insomnia symptoms is associated with suicide risk independently of current symptoms of insomnia, depression, anxiety, or PTSD. 177 Furthermore, a random-digit-dial study of US adults showed that insomnia complaints were more strongly associated with suicide ideation than ratings of poor sleep, 197 and other studies have shown higher levels of suicide ideation among participants whose sleep concerns did not improve over the course of the treatment (independently of baseline sleep concerns, depressive symptoms, and suicide ideation) 198 and that sleep dissatisfaction can explain a significant portion of the relation between insomnia and the wish to die. 129

DBAS
In addition to nightmares, it has been proposed by McCall et al that DBAS may explain part of the relation between insomnia symptoms and suicide ideation due to the indirect effect of DBAS approaching significance in their cross-sectional sample of adults with depression. 43

Nightmares
Nightmares have received increased attention over the past decade with regard to their potential role as a mediator of the insomnia-suicide relation. Research has shown that nightmares are associated with insomnia symptoms, 43,199 independently predictive of suicide ideation in cross-sectional samples, 38,89,165 and that the relation between insomnia symptoms and suicide ideation becomes nonsignificant when including nightmares 43 and holding symptoms of anxiety, depression, and PTSD constant. 38 Furthermore, coexisting insomnia symptoms and nightmares have been shown to increase the risk for a future suicide attempt in an adult sample of psychiatric outpatients, 58 potentially supporting the supposition that nightmares may be one reason for middle insomnia. 60

Hyperarousal
Insomnia and PTSD-related nightmares have both been proposed as conditions of hyperarousal, 196 and hyperarousal during sleep has been proposed as a neurobiological correlate of suicide ideation in adults with major depressive disorder. 200 Recent studies have examined the role of hyperarousal in suicide risk, 201 and a study of US military personnel showed a stronger relation between insomnia symptoms and suicide ideation among those with heightened levels of agitation. 35 A dated study of suicide-attempt survivors, however, showed that 89% of attempt survivors denied experiencing agitation in the week prior to their suicide attempt. 21

Thwarted belongingness
Recent research has examined the indirect effect of thwarted belongingness, a variable included in the suicidal desire component of the IPTS, 202 on the relation between insomnia symptoms, suicide ideation, and suicide risk, showing that the variable accounts for a significant portion of the relation between insomnia and suicide ideation across multiple samples. 39,47,56 However, it should be noted that the indirect effect of thwarted belongingness on the relation between insomnia and suicide risk was not significant when adjusting for depressive symptoms, 47 and prior research has shown that the IPTS or thwarted belongingness alone fail to mediate the relation between insomnia symptoms and past suicide attempts cross-sectionally. 74 Furthermore, the results from Hom et al 47 specifically showed that the indirect effect of thwarted belongingness on the relation between insomnia and suicide ideation was more meaningful for those with lower levels of perceived burdensomeness. This finding potentially conflicts with the IPTS as originally proposed, 202 and suggests that the IPTS may not be able to explain how insomnia symptoms lead to increased suicide risk across all samples, which is consistent with previous research. behavioral therapy for insomnia (CBT-I) is one insomnia treatment that appears promising for mitigating suicide risk in insomnia patients. CBT-I is currently recommended as the initial treatment for chronic insomnia in adults, 85 and is associated with significant reductions in suicide-related thoughts from pre-to posttreatment. 18,84,86 Researchers have continuously called for greater empirical focus on investigating the effects of insomnia treatment on mitigating suicide risk. 33,64,[79][80][81][82][83] We also argue that such research is needed, especially since there has not been enough research to establish adequate efficacy and because prescription sleep medications (eg, sedative-hypnotics) have been associated with suicide ideation 34 and suicide, 57,87 and were a stronger predictor of past-year suicide ideation and nonfatal suicide attempts than insomnia symptoms in a general US population sample. 88

Sleep-related breathing disorders
The number of outpatient visits resulting in a diagnosis of sleep apnea increased 442% from 1999 to 2010, 4 but the prevalence of suicide-related thoughts and behaviors among sleep-apnea patients is currently unknown. Studies have shown mixed findings regarding the relation between sleep apnea and suicide risk. Sleep-related breathing symptoms were not related to suicide ideation in an adult outpatient sample, 89 but have been associated with suicide ideation and suicide-attempt planning in a general population sample of adults 90 and greater depression severity and suicide risk in a sample of female sexual assault survivors (compared to those without sleep-related breathing symptoms). 91 In the latter study, the greatest suicide risk was shown for those participants presenting with concurrent sleep-related breathing symptoms and a sleep-movement disorder. It is unknown if a coexisting psychiatric illness or sleep disorder would explain the discrepant findings between these two studies. For instance, research has shown that 39%-61% of obstructive sleep-apnea patients also have insomnia symptoms. [92][93][94] It remains to be seen if sleep-related breathing symptoms are associated with suicide-related thoughts and behaviors independently of other sleep or psychiatric disorders.
Results from a case report and a study of adults assessed for obstructive sleep apnea suggest that treatment of sleeprelated breathing via nasal continuous positive airway pressure may resolve suicide-related thoughts. The case study 95 focused on a 74-year-old male presenting with severe depression, excessive daytime sleepiness, and suicide ideation with a suicide-attempt plan, and noted that treatment of sleep-apnea symptoms appeared to lead to the resolution of depressive symptoms (a result supported by prior research) 96 and suicide risk. In the study of adults assessed for obstructive sleep apnea, Edwards et al 97 showed that among the subset of the sample that reported suicide ideation at baseline, none endorsed suicide ideation after 3 months of continuous positive airway-pressure adherence.
More research is needed, especially studies using objective sleep measures, in order to ascertain suicide risk among patients with central or obstructive sleep apnea. Additionally, little is known about the contribution of snoring to suicide risk. One study showed higher suicide risk for South Korean adolescents who reported snoring compared to those who denied snoring, 55 though snoring did not appear independently to contribute to suicide risk when adjusting for other sleep-related variables. Lastly, it is unknown if other sleeprelated breathing disorders, such as sleep-related hypoventilation disorders or catathrenia, confer an increased suicide risk.

Central disorders of hypersomnolence Narcolepsy
When compared to the general population, patients with narcolepsy have an increased mortality risk across all agegroups. 98 Although few studies have examined the prevalence of suicide risk, results from the Burden of Narcolepsy Disease study showed that patients with a narcolepsy diagnosis were at increased risk of suicide-related behavior compared to matched controls, including increased odds of having a depressive or anxiety disorder. 99 Also, Ohayon 98 discussed results from an unpublished longitudinal study of narcolepsy patients 100 showing a sevenfold-greater risk for suicide among those with narcolepsy. More research is needed in order to understand better whether aspects of narcolepsy confer greater suicide risk or if the risk can be explained by a concurrent psychiatric illness (eg, depression). Furthermore, it is unknown if narcolepsy treatment can allay suicide risk.

Idiopathic and other hypersomnias
Hypersomnia symptoms have been associated with suicide ideation 101 and nonfatal suicide attempts 102 in adolescents. Hypersomnia symptoms have also been associated with increased suicide risk 103 and higher odds of past-year and lifetime nonfatal suicide attempts in a sample of depressed adults. 104 Hypersomnia has also been associated with pastyear suicide ideation and suicide-attempt planning when experienced concomitantly with insomnia symptoms. 104 Goldstein et al 23 found a higher prevalence of hypersomnia in adolescent suicide decedents compared to community controls. Based on information from decedent informants, 15% of adolescent suicide decedents experienced hypersomnia in the week preceding suicide and 16% experienced hypersomnia around the time of suicide. Additionally, it was more likely for the suicide decedents' hypersomnia symptoms to worsen in the week preceding suicide (compared to controls). 23 However, a psychological autopsy study of adult suicide decedents showed that the concurrent presence of hypersomnia with other depressive symptoms (ie, fatigue, impaired concentration/indecisiveness, and weight/appetite gain) was associated with decreased risk for suicide. 62 More research is needed to determine if hypersomnia increases suicide risk independently of concurrent psychiatric illness symptoms, and if so, whether management of hypersomnia can mitigate suicide risk.

Kleine-Levin syndrome
Very few studies have investigated the prevalence of suicide risk in patients with Kleine-Levin syndrome (KLS). One study of 108 KLS patients showed that 3.7% of the sample endorsed thoughts of attempting suicide, 105 but no other studies to our knowledge have examined suicide-specific variables and potential confounders (eg, major mood disorder) 106 in samples of KLS patients. Reviews of the general KLS literature conclude that suicide attempts are rare among those with KLS. 107,108 Even so, Arnulf et al suggested monitoring suicide risk in patients with KLS. 108

Insufficient-sleep syndrome
A study of South Korean adolescents with behaviorally induced insufficient-sleep syndrome (BISS) showed that those with BISS had higher scores on measures of depression and suicide risk than adolescents without BISS and with sufficient sleep, adjusting for age and sex. 55 Significant differences in regard to suicide risk remained when adjusting for depressive symptoms, and oversleeping on the weekend were associated with suicide risk independently of depression, insomnia symptoms, snoring, and daytime sleepiness, suggesting that chronic sleep deprivation may be an independent suicide risk factor. These results are consistent with other studies showing that Korean adolescents who slept longer on weekends were at greater risk of reporting a recent suicide attempt or self-injury 109 and Chinese adolescents with greater suicide risk reporting short sleep duration during weekdays, increased sleep compensation and long sleep duration during weekends, and greater frequency of daytime napping. 110 Furthermore, a more recent study of US adults showed that both long and short sleep durations were associated with suicide risk. 111 Short sleep duration (defined in various studies as ≤4, ≤5, ≤6, or <8 hours of sleep) has been associated with increased odds of suicide risk in samples of Taiwanese adults, 57 US adolescents, 102,112,113 and US Army soldiers following redeployment. 114 Similar findings have been shown in samples of South Korean, 115 Taiwanese, 116 and Chinese adolescents 117 (all were independent of depressive symptoms). Short sleep duration has also been associated with past 12-month suicide ideation and nonfatal suicide attempts in a South Korean adolescent sample (the relation was weaker for depressed adolescents vs nondepressed) 118 and in a community sample of US adults after adjusting for symptoms of depression, bipolar disorder, panic attacks, alcohol and substance dependence, and antisocial personality disorder. 119 Increased risk of suicide ideation has also been shown among rural Chinese adolescents, 120 European adolescents, 121 US adolescents, 122 and South Korean adolescents 123 and adults 124-126 reporting short sleep duration, and one study showed that male Spanish suicide-attempt survivors were more likely to report shorter sleep duration than psychiatric inpatient controls who denied a suicide-attempt history 127 (short sleep duration was also associated with intent to attempt suicide among female attempt survivors, consistent with recent research). 128 The potential importance of sleep duration in the development of suicide risk was emphasized in two longitudinal studies. The first showed that the wish to die among a small sample of French adults 1 month after an index suicidal crisis increased significantly with every lost hour of sleep, 129 which is consistent with cross-sectional adolescent research showing that a 1-hour reduction in weekday sleep was related to an increased risk of hopelessness, suicide ideation, and suicide attempts (risk was higher for males). 130 In a sample of adolescent monozygotic twins, the second study showed that a late bedtime and short sleep duration predicted future development of depressive and anxiety symptoms and suicide and self-injury risk (independently of bedtime regularity and genetic and shared environmental factors). 131 Though it should be noted that inconsistencies exist, as one study showed that short sleep duration (ie, ≤5 hours of sleep) and nonrestorative sleep were associated with last-12-month suicide ideation, but not last-12-month suicide attempts in a sample of South Korean adolescents when adjusting for demographics, substance use, and depressive symptoms. 132 More recent research showed depressive symptoms moderating the sleep duration-suicide risk relation among Chinese adolescents, 133 and a study of South Korean adolescents showed that short weekday sleep duration was not associated with last-12-month suicide ideation or suicide 219 Suicidality in sleep disorders attempts when included in regression models with demographic variables, weekend sleep duration, and depressive symptoms. 109 Furthermore, a dated study showed that risk of death by suicide was not higher among those with short or long sleep duration. 134 More research is needed on exploring for mechanisms or confounders of the suicide-risk relation that both insufficientsleep syndromes and sleep duration have demonstrated. Additionally, it is unknown if addressing sleep duration alone can alleviate risk of attempting suicide. Some researchers propose that for adolescents, parents may be able to increase sleep duration and decrease adolescent risk for depression and suicide ideation by setting earlier bedtimes 122 and restricting electronic media access after bedtime. 135

Excessive daytime sleepiness
Although excessive daytime sleepiness does not qualify as a sleep disorder, the prevalence of daytime sleepiness (a symptom present across several sleep disorders, eg, insomnia and narcolepsy) was shown to be higher among a sample of adolescent suicide decedents from the US when compared to community controls (13% vs 5%). 23 Cross-sectional research has shown that excessive daytime sleepiness is associated with depression-symptom severity and suicide ideation and attempts in patients with depression, 60 with those reporting more sleepiness showing higher self-reported levels of suicide ideation than those with less daytime sleepiness. 136 However, a follow-up study showed that insomnia symptoms but not excessive daytime sleepiness were associated with suicide ideation when adjusting for demographic factors, depression duration, insomnia symptoms, and antidepressant/ hypnotic-medication use. 44 In addition, results of the followup study showed that those with insomnia symptoms had significantly higher scores on the suicide-attempt planning and active and passive suicide-ideation components of the Beck Scale for Suicide Ideation 137 than those with excessive daytime sleepiness. Also, no association was shown between excessive daytime sleepiness and suicide risk in a sample of adults admitted to an emergency department who presented with passive or active suicide ideation. 129

Circadian rhythm sleep-wake disorders
Circadian rhythm sleep-wake disorders (CRSWDs) have not received much attention in terms of examining the prevalence of suicide-related thoughts and behaviors and identifying how specific sleep-wake disorders may impact one's risk for attempting suicide. The few studies that have been done have shown mixed results. Adults with bipolar disorder and a CRSWD had a greater likelihood of having a family history of suicide than participants without a CRSWD, 138 though an earlier study of adults with bipolar disorder showed no differences between participants with delayed sleep phase, hypersomnia, insomnia, or no sleep problems with regard to the number of past suicide attempts. 139 Furthermore, the prevalence of circadian reversal was not found to be significantly different in adolescent suicide decedents when compared to community controls. 23 A more recent study of US adolescents showed that circadian reversal was associated with lifetime suicide attempts, but not suicide ideation or nonsuicidal self-injury, when adjusting for age, sex, depressive symptoms, insomnia symptoms, nonrestorative sleep, and hypersomnia. 140 Research focusing on chronotypes have shown associations between an "eveningness" preference and suiciderelated thoughts [141][142][143][144] and behaviors (ie, lifetime and violent suicide attempts). 145,146 An eveningness preference has also been associated with elevated scores on measures of depressive symptoms, hopelessness, and defeat. 147 Additionally, an eveningness preference coupled with short sleep duration was associated with greater odds of suicide ideation when adjusting for demographics, hopelessness, and past-month sleep problems due to bad dreams. 141 Although associations have been shown between eveningness and suicide risk (current and lifetime), 110,146 some studies have shown that eveningness was not associated with current (when adjusting for depressive symptoms) 148 or lifetime suicide risk. 147 Research focused on suicide-related variables has also been fairly absent among samples of participants experiencing jet lag or difficulties due to shift work. One of the few studies that examined shift work and suicide ideation showed an interaction of day shift and depressive symptoms for suicide ideation among female police officers, with female officers reporting higher levels of depression showing an increase in suicide ideation as day-shift hours increased. 149 Among male police officers with higher levels of depression, suicide-ideation levels increased as the frequency of shift changes increased. It is generally unknown if treating CRSWDs can mitigate suicide risk, though one small study of depressed inpatients with suicide ideation showed a significant reduction in suicide risk following triple-chronotherapy treatment. 150

Parasomnias
Most of the research published on parasomnias and suicide has focused on parasomnia conditions generally confined to REM sleep (ie, nightmares). Among the non-REM-related parasomnias, sleep paralysis was suggested as a precipitant to suicide-related behavior in one case study 151 and suicide ideation was more likely to be endorsed among Taiwanese adolescents with concurrent sleep terrors and sleepwalking in a small case-control study. 152 Sleepwalking has been suggested as a possible contributor to accidental nonfatal and fatal suicide attempts, [153][154][155][156] but most of the research conducted in this area has been case studies. Similar concerns about accidental self-injurious behavior have been reported for patients with REM sleep-behavior disorder, 157 with a retrospective chart review of sleep-clinic patients showing an increased prevalence of "acting out dreams" in adults with suicide ideation compared to those without ideation. 158 Some researchers have suggested that the use of hypnotics (ie, zolpidem and eszopiclone) could heighten self-injury or suicide risk due to inducing parasomnia behavior that was absent prior to hypnotic use. [159][160][161][162] Nightmare disorder Research examining nightmares and suicide risk has focused more on the experience of nightmares versus the presence of a nightmare-disorder diagnosis, and few studies have examined the prevalence of suicide-related variables among those with nightmares ( Table 4). The experience of bad dreams and nightmares has been associated with suicide ideation and suicide risk in samples of youth, 117,120,163,164 adults, 22,43,45,58,75,89,[165][166][167][168][169][170][171][172] and older adults. 52,173 Moreover, nightmares have been independently associated with lifetime, 58,74,164,169 recent, 58,76,171,174 and subsequent suicide attempts, 58,75 including suicide. 175,176 Importantly, the variance in suicide-related variables explained by nightmares across multiple studies has been independent of several potential confounders, including age, sex, anxiety and depressive symptoms, and the interpersonal-psychological theory of suicide. 22,74,75,117,171,173 Nightmares have also independently predicted repeated suicide attempts in a sample of suicide-attempt survivors, 75 Table 4 Prevalence of suicide-related thoughts and behaviors among participants with nightmares and proposed mechanisms of the nightmare-suicidality relation

Prevalence
In a study of adults with a history of trauma, 62% who experienced nightmares during the past month reported suicide ideation, suicide attempt planning, or a nonfatal suicide attempt, 172 and about 96% of adult participants in a separate study who reported melancholic features and a suicide attempt during a current depressive episode reported experiencing nightmares at least twice a week. 76 Among adolescents who often experience nightmares, 40% reported suicide ideation (compared to 13% of those who denied nightmares); the percentage increased to 50% for those who reported experiencing nightmares very often. 28

Proposed mediator Summary of research Insomnia
Some studies have suggested that insomnia symptoms may explain the relation between nightmares and suicide ideation or a suicide-attempt history. An 8-year observational study of adults with schizophrenia-spectrum disorders showed that nightmares were associated with a history of suicide attempts, but only baseline insomnia symptoms were associated with an increased risk of suicide attempt at follow-up, with coexisting insomnia symptoms and nightmares conferring an elevenfold-increased risk for a future suicide attempt. 59 Additionally, among a primary-care clinic sample of older adults, insomnia symptoms had an indirect effect on the association shown between nightmares and suicide ideation, 52 and nightmares and insomnia symptoms predicted suicide ideation after 1 month (independently of baseline suicide ideation) in an online sample of adults, but only insomnia symptoms independently predicted 1 month suicide ideation. 203

Negative affect
Hochard et al 204 showed in a university sample that nightmares significantly increased the risk for future self-harm ideation and behaviors and that negative affect had an indirect effect on this relation. However, the direct effect of nightmares on self-harm risk was larger than the indirect effect, suggesting that negative affect cannot explain the majority of this relation. 182

Defeat, entrapment, and hopelessness
Littlewood et al 172 showed that the relation between nightmares and suicide risk is partially mediated by feelings of defeat, entrapment, and hopelessness. Notably, the effects of this multiple-mediation model remained after adjusting for insomnia symptoms and depression diagnosis.

Nightmare distress
A cross-sectional study of undergraduates showed that nightmare frequency had an indirect effect on suicide ideation through nightmare distress, independently of insomnia symptoms. 205 In a moderated mediation analysis with sex as the moderator, results showed that the indirect effect of nightmare distress on the nightmare frequency-suicide ideation relation was significant only for women.

IPTS
Three studies have examined whether the IPTS could explain the nightmare-suicidality relation, with two studies showing that the full IPTS could not fully account for this relation 74,173 and one showing that neither thwarted belongingness nor perceived burdensomeness could fully account for the nightmare distress-suicide ideation relation. 206 However, it should be noted that in the latter study, perceived burdensomeness partially mediated the nightmare distress-suicide ideation relation, with a moderated mediation showing that this effect was significant only for women. and studies among adults and older adults have shown that the chronicity of nightmares may increase the risk of attempting suicide. 173,177 However, in an online sample of US adults, nightmare frequency differentiated multiple and single suicide attempters independently of nightmare chronicity, nightmare severity/distress, and symptoms of depression, posttraumatic stress disorder, and insomnia in a sample of US adults (Speed et al, under review, 2017), suggesting that the number of nightmares experienced within a specific time frame may have greater relevance to predicting repeat suicide attempts than the length of time nightmares have been experienced. Additional inconsistencies have been observed. For instance, nightmares were inversely associated with suicide ideation in a sample of Canadian military personnel and veterans 178 and were not more frequent among depressed adolescent outpatients reporting suicide ideation or suicide intent. 54 Also, two separate studies of adolescents failed to show an association between nightmares and suicide-related thoughts and behaviors, 19,179 though one of the adolescent studies showed that parent-reported nightmares were not associated with suicide attempts 179 and recent research showed discrepancies between parent and child reports on measures of distress, with more distressed children showing a greater likelihood of endorsing nightmares and suicide ideation than parents. 180 Research has also shown an absence of suicide-related behavior among treatment-seeking patients with posttraumatic stress disorder and nightmares, 181 and researchers suggest that stronger risk factors may exist that can explain the nightmare-suicide relation. 176 As noted by Nadorff et al, 182 progress is being made in the search for mechanisms that can explain the process whereby nightmares confer greater risk for suicide (see Table 4 for a review of mediator studies), but the management of suicide risk via nightmare treatment is one area that may deserve greater exploration. Results from an uncontrolled trial 183 showed that imagery rehearsal therapy may resolve both nightmares and suicide ideation, with a case study suggesting that prazosin 184 may also be effective in achieving similar outcomes. Both of these treatments have demonstrated efficacy in treating nightmares. 185

Sleep-related movement disorders
Patients with restless-leg syndrome (RLS) appear at risk of developing depression, 167 insomnia, anxiety, and pain disorders, 186,187 conditions that have been associated with suicide risk. 23,188,189 Separate studies showed that 21% 190 and 38% 191 of sampled RLS patients endorsed thoughts of suicide. Depressed RLS patients are also more likely than controls to blame RLS symptoms entirely for their depressive symptoms, sleep problems, and suicide-related thoughts. 191 Additional research is needed among RLS patients, as it is unknown if concurrent psychiatric illness or insomnia symptoms can explain the increased prevalence of suicide ideation noted. There also is a paucity of research focused on investigating suicide risk among other sleep-related movement disorders. This includes periodic limb-movement disorder, sleep-related leg cramps, sleep-related bruxism, sleep-related rhythmic movement disorder, propriospinal myoclonus at sleep onset, and sleep-related movement disorder due to a medical disorder or medication/substance use.

Conclusion
This narrative review has revealed multiple research avenues with regard to sleep disorders and suicide risk. Although the impact of some sleep-disorder symptoms on suicide risk has been investigated extensively (ie, insomnia), we still know very little about the prevalence of suicide risk across sleep disorders and the impact these disorders have on suicidality independently of known risk factors (eg, depression). We also know little about whether managing sleep problems via evidence-based treatments can mitigate risk for suicide, and if effective, what populations seem to benefit most from such management. At this point, we can make the argument that sleep disturbances may be a marker of distress and suicide risk 127 and that multiple concurrent sleep problems may confer greater risk for suicide than single sleep problems. 66,141 Mechanisms explaining the relation between sleep disorders and suicide remain needed, including more research rigorously examining the efficacy of sleep-disorder treatments on assuaging suicide risk. Research examining the impact of sleep disorders on suicide risk has been circumscribed to a few sleep-disorder symptoms, with much less known about the prevalence and management of suicide risk across all sleep disorders. Therefore, opportunities abound for current and prospective sleep and suicide researchers.

Disclosure
The authors report no conflicts of interest in this work.