In humans, normal sleep is punctuated with changes in brain wave activity and other physiologic parameters. Sleep is generally divided into two broad stages: nonrapid eye movement (NREM) and rapid eye movement (REM). NREM sleep is further subdivided into distinct stages based on EEG changes.
Healthy adults progress through four to six cycles of alternating NREM and REM sleep each night, descending into deep sleep, rising to REM sleep, and then descending again into the deeper stages. Each cycle lasts approximately 90 to 100 minutes. NREM comprises 75-90% of adult sleep time, with REM accounting for the remaining 10-25%.
Nonrapid Eye Movement Sleep
NREM sleep consists of four stages—numbered 1 through 4—characterized by decreasing levels of consciousness. EEG analysis shows slowed activity in the deeper stages.
- Stage 1 (also referred to as drowsiness or “pre-sleep”): EEG shows a dropout of alpha activity (the normal “awake” pattern) and increasing 4- to 8-Hz theta and beta activity. Other EEG patterns may be noted (positive occipital sharp transients, K complexes, V waves, etc.). Slow rolling eye movements (SREMs) are seen. Muscular activity—limb movement, rolling, and twitching—is still present.
- Stage 2: The predominant stage of sleep in adults, accounting for 45-55% of total sleep time. EEG reveals “sleep spindles” (short bursts of 12-16 Hz activity) and K complexes. SREMs disappear. Blood pressure may fluctuate. Muscular activity slows.
- Stages 3 and 4 (slow-wave sleep or SWS): Classically correlated with delta waves (1/2 to 2 Hz) on EEG. Stage 3 is defined by delta activity occupying 20-50% of the time; stage 4 is delimited by more than 50% delta wave activity. Muscular activity is decreased in both stages. Individuals in SWS (“deep sleep”) are harder to awaken; SWS is perceived by most people to be high-quality sleep.
REM (Paradoxical) Sleep
REM sleep is defined by three criteria:
- Rapid eye movements: rapid bursts of back-and-forth eye motion
- Muscle atonia: “paralysis” of skeletal muscles
- EEG desynchronization: EEG waveform is more typical of lighter sleep stages (theta and beta activity)
Additionally, most dreams occur during REM sleep. Respiratory rate and depth fluctuate dramatically.
REM sleep typically follows a cycle of NREM sleep, with the majority of REM usually occurring late in the sleep cycle (toward morning).
(Adapted from Benbadis S, Rielo D. Normal Sleep EEG. eMedicine Electroencephalography Atlas Sept. 11, 2008)
Changes in Sleep Patterns with Aging
Infants may spend up to 20 hours sleeping each day; approximately 50% of that time is spent in REM sleep. With age, people sleep less and spend less time in deep sleep and REM sleep.
In advanced age, sleep is often abbreviated and interrupted. REM sleep time shortens, and stage 4 sleep may disappear altogether. Such changes in sleep patterns with aging may be responsible for fatigue, daytime sleepiness and other complaints. (The Merck Manual, 18th Edition. 2006:215-16)
Sleep Fills a Physiological Need
Humans spend nearly one-third of their lives sleeping. According to Matthew Walker, director of the Sleep and Neuroimaging Lab at the University of California, Berkeley, all animals, from the elephant to the fruit fly, need varying amounts of sleep. (The Science Of Sleep: Leslie Stahl Explores the Latest Findings in Sleep Research. CBS News, 60 Minutes. June 15, 2008)
While sleep requirements vary widely from species to species (and some animals can apparently sleep with half their brains at a time), sleep seems to be a universal need. Given the vulnerability of an animal when it is unconscious, sleep must represent a critical function for survival and wellbeing.
