{'fraction_non_alphanumeric': 0.019708167519423916, 'fraction_numerical': 0.0053060451013833615, 'mean_word_length': 5.283333333333333, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Can habitual sleep duration harbor sleep debt?

IN THIS ISSUE, KLERMAN AND DIJK1 REPORT AN EXPERIMENT THAT FOCUSES ON HOW TO INTERPRET DIFFERENCES AMONG PEOPLE IN HABITUAL BEDREST duration (HBD) using daytime sleep propensity and a sleep extension paradigm to evaluate the relationship between HBD and purported sleep need. There is considerable scientific evidence that sleep debt can develop in a dose-response manner relative to reduced time in bed, and that the effects can be cumulative and include significant functional consequences, especially in objective measures of alertness.2-6 But we cannot say a priori whether naturally occurring shorter-duration HBDs are a sign of sleep debt. That is, we do not know to what extent variation among people in habitual sleep duration found in any given population of healthy individuals is primarily a reflection of inter-individual variability in biological sleep need, or to what extent it reflects variation in sleep debt resulting from a forced reduction in sleep duration that masks basal sleep need.7A The problem making this discrimination becomes evident when attempting to explain what is perhaps one of the most ubiquitous features of human sleep-wake behavior—namely the extension of sleep duration on weekends or holidays or days off. Is the extra sleep (nocturnal extension and diurnal naps) on days when work or domestic duties are not mandated indicative of a need to compensate sleep debt by obtaining extra (recovery) sleep? Or is it merely a social decision to consume sleep we don’t need (i.e., what was originally labeled “optional sleep” but now is referred to as “adaptable sleep”8)? In other words, what is the biological significance of an ability to obtain extra sleep relative to habitual sleep duration? Klerman and Dijk1 took an approach to studying habitual sleep duration similar to the way in which Kleitman used the phrase “sleep debt” to characterize what results when one reduces nocturnal sleep time, then attempts to reverse the effects by extending sleep to “liquidate the debt” (p. 317).9 This original conceptualization of sleep debt to explain periodic extension of sleep duration remains relevant to both the fundamental question of how much sleep people need to avoid a cumulative sleep debt, and which aspects of sleep physiology are essential for recovery of healthy waking neurobehavioral and physiological functions. The experimental effects of sleep extension have received scant attention, but the few studies that have been published suggest extended sleep duration can normalize wakefulness10 and metabolic responses,11 and perhaps even expose the basal sleep need of an individual.12 The experimental results of Klerman and Dijk1 provide some of the first evidence that inter-individual variation in well-documented habitual bedrest duration may harbor substantial sleep debt, rather than simply reflecting natural biological variability in basal sleep need. These investigators quantified habitual bedrest duration in 17 healthy young adults using daily diaries, timestamped call-ins, and actigraphy over a 3-week period, and then evaluated the relationship between HBD on the final week and daytime sleepiness (MSLT) after 1 night in the laboratory. They found that individuals with shorter habitual bedrest durations during the 3 weeks prior to laboratory testing fell asleep more quickly and frequently during the MSLT (often less than 5 minutes) than did those with a longer HBDs, suggesting that sleep debt—as determined by daytime sleep propensity—increased as habitual bedrest duration decreased. The finding is consistent with other evidence that habitual short sleepers tend to have a higher homeostatic sleep pressure than longer sleepers,13 and that they would benefit from increased time in bed for sleep.14 This is an important observation, but it does not tell us whether these short sleepers are able to resist the elevated sleep drive they experience (i.e., have a greater tolerance to sleep debt), or whether they are vulnerable to the sleep debt and experience greater functional deficits and risks associated with sleep debt. Distinguishing between these two alternatives would require other measures of waking functions (e.g., cognitive performance, metabolic outcomes). If healthy adults with short-duration HBD carry a higher homeostatic sleep drive because they are resistant to sleep loss, then there should be no cognitive performance (or metabolic) changes over time at habitual bedrest duration. On the other hand, if the higher sleep debt of short-duration HBD subjects manifests in waking functions, then there should be clear evidence of cognitive performance (or metabolic) changes over time at typical HBD. In this eventuality, the question is how these individuals maintain such reduced sleep durations over the long term. Although the MSLT data of Klerman and Dijk1 are consistent with the hypothesis that shorter-duration HBD in healthy young adults may harbor sleep debt that reduces daytime alertness, it is also possible that short-duration HBD individuals may demonstrate ease of falling asleep on the MSLT, but experience no difficulty remaining awake and performing alertly in the face of Can Habitual Sleep Duration Harbor Sleep Debt?