Human sleep: An overview

Abstract

The Rechtschaffen and Kales system for scoring sleep states distinguishes a waking state, nonrapid eye movement (NREM) sleep, and rapid eye movement (REM) sleep. From the polysomnographic point of view, four stages are conventionally distinguished during NREM sleep. The young adult spends 20-28 of a night's sleep in REM sleep, 4-5 in stage 1, 46-50 in stage 2, 6-8 in stage 3, and 10-16 in stage 4. A range of differences in average sleep length has been established in young adults. In this respect, subjects have been characterized who sleep significantly less (short sleepers) or more (long sleepers) relative to a group norm. Short sleepers spend less time in stages 1, 2, and 3 sleep, whereas long sleepers spend more time in stage 2 sleep and REM sleep. On the basis of behavioral and polysomnographic aspects, three sleep states have been defined in the newborn infant: quiet sleep (the precursor of adult NREM sleep), active-REM sleep (the precursor of adult REM sleep), and indeterminate sleep. Total sleep time attains its highest levels in neonates and young infants (16-17 h). The fastest decrease in sleep length is observed in the 6-month-old infant (13-14 h) and is mainly related to the diminution of REM sleep during daytime. Compared to the infancy, changes of sleep duration during early childhood occur at a slower pace. The total amount of sleep decreases to 12 h by 4 years and to 8-10 h by 10 years. A series of neural structures involved in the occurrence of waking, NREM sleep, and REM sleep have been characterized in the central nervous system (CNS). In addition, a number of neurotransmitters have been described that function to promote waking (acetylcholine, noradrenaline, dopamine, serotonin, histamine, glutamate, and orexin), NREM sleep (Γ-aminobutyric acid, gallanin, and adenosine), and REM sleep (acetylcholine). © 2008 Humana Press, Totowa, NJ.