neurologic disorders
Approach to the Patient With a Sleep or Wakefulness Disorder
Pathophysiology
There are 2 states of sleep, each marked by characteristic physiologic changes: NREM sleep constitutes about 75 to 80% of total sleep time in adults. Heart rate and body temperature tend to decrease. NREM sleep consists of 3 stages (N1 to N3) in increasing depth of sleep. Slow, rolling eye movements, which characterize quiet wakefulness and early stage N1 sleep, disappear in deeper sleep stages. Muscle activity also decreases. Stage N2 sleep is characterized by K complexes and sleep spindles on the EEG (see figure Nonrapid eye movement (NREM) EEG ). Stage N3 is referred to as deep sleep because arousal threshold is high; people may perceive this stage as high-quality sleep. Rapid eye movement (REM):REM sleep follows each cycle of NREM sleep. It is characterized by low-voltage fast activity on the EEG and postural muscle atonia. Respiration rate and depth fluctuate dramatically. Most dreams occur during REM sleep. Normally, 20 to 25% of sleep is REM. Progression through the 3 stages, typically followed by a brief interval of REM sleep, occurs cyclically 5 to 6 times a night (see figure Typical sleep pattern in young adults ). Brief periods of wakefulness (stage W) occur periodically. These EEG tracings show characteristic theta waves, sleep spindles, and K complexes during stages 1 (N1), 2 (N2), and 3 (N3) NREM sleep. This figure includes an EEG tracing (showing characteristic sawtooth waves) and an eye tracing (showing rapid eye movements), which occur during REM sleep. In the bottom figure, the arrows represent sharply peaked conjugate eye movements from the right and left eyes during REM sleep. Individual sleep requirements vary widely, ranging from 6 to 10 hours/24 hours. Infants sleep a large part of the day; with aging, total sleep time and deep sleep (stage N3) tend to decrease, and sleep becomes more interrupted. In older people, stage N3 may disappear. These changes may account for increasing EDS and fatigue with aging, but their clinical significance is unclear. Rapid eye movement (REM) sleep occurs cyclically throughout the night every 90–120 min. Brief periods of wakefulness (stage W) occur periodically. Sleep time is spent as follows: Stage N1: 2–5% Stage N2: 45–55% Stage N3: 13–23% REM: 20–25% 