In the awake state, individuals may either be awake with their eyes open or awake with their eyes closed. In both, the brain activity is usually low voltage, high frequency waves. When the eyes are open, individuals are alert, there is rapid movement of the eyes (with blinking), and muscle activity is relatively high. When the individual is awake with eyes closed, the frequency of brain activity decreases, eye movements are slow and rolling, but the muscle activity remains relatively high.1,2
NREM sleep – also known as drowsiness – is the first step in a normal sleep cycle lasting about 5 or 10 minutes. This is a light sleep during which it is easy to drift in and out of sleep, the frequency of brain activity decreases, the eyes move slowly, and muscle activity slows.1,3
NREM sleep follows Stage 1. Eye movement stops and brain waves become slower with an occasional burst of rapid brain waves.3 In Stage 2, the heart rate slows down and the body temperature decreases.4
NREM sleep – the first stage of SWS – follows Stage 2 and is associated with very slow (delta) brain waves interspersed with smaller, faster waves.
NREM sleep follows Stage 3 and is associated exclusively with slow wave delta waves. In Stages 3 and 4 NREM sleep, there is no eye movement or muscle activity and awakening is difficult. SWS occurring in these two stages is considered an accurate marker of sleep depth, and is thought to correspond with the deepest state of sleep.5 Hence, SWS also known as ‘deep sleep’ and is considered to be the most restorative sleep, i.e., important in the quality of sleep and the feeling of refreshment that is experienced by an individual after sleep. Following a period of SWS, there is a brief return to Stage 2 sleep, swiftly followed by initiation of REM sleep3,4
REM sleep occurs 70–90 minutes after sleep onset and just after the brief return to Stage 2 following SWS. In the REM period, breathing becomes more rapid, irregular and shallow, eyes jerk rapidly and limb muscles are temporarily paralysed. Additionally, brain waves increase to the pattern of an awake state, heart rate increases, blood pressure rises, males develop an erection, and body temperature falls. This is the stage of sleep during which most dreams occur.3 Periods of REM sleep normally last for 5–10 minutes but, with each successive sleep cycle, the duration of REM sleep increases and NREM decreases.6
Arousals and awakenings are a normal part of the sleep cycle in most healthy adults, occurring mostly in Stage 1 and Stage 2 sleep, and not usually observed during SWS. During REM sleep, bursts of alpha activity are observed and, so, for arousals to occur in this stage, a simultaneous increase in EMG activity is required. The threshold to arousal increases as sleep deepens.7
Most adults have 5–7 sleep cycles a night, each lasting approximately 90– 110 minutes, and composed of a segment of NREM sleep followed by REM sleep.1,7 Overall, in a healthy young adult, about 50% of the night is spent in Stage 2 NREM sleep, and 20–25% in REM sleep.6 Within each cycle, there may be intermittent awakenings. SWS predominates during the first one third of the night, while REM sleep increases during the last few hours of sleep. Generally, as the night progresses, the length of REM sleep in each cycle increases.1,7
References:
1. Berry RB, Geyer JD, Carney PR. Introduction to sleep and sleep monitoring – the basics. In: Carney PR, Berry RB, Geyer JD, eds. Clinical Sleep Disorders.© Lippincott Williams & Wilkins, Philadelphia, USA, 2005: 3–26.
2. Armon C, Roy A, Nowack WJ, et al. Polysomnography: overview and clinical application. eMedicine, 2007.
3. SleepDex – Resources for Better Sleep: Stages of Sleep. www.sleepdex.org/stages.htm. Accessed January 2008.
4. Understanding Sleep: Sleep Needs, Cycles, and Stages. www.helpguide.org/life/sleeping.htm. Accessed January 2008.
5. Kandel ER, Schwartz JH, Jessell TM. Principles of neural science.© The McGraw-Hill Companies Inc, 2000.
6. Rechtschaffen A, Siegel, J. Sleep and dreaming. In: Principles of neuroscience. Fourth Edition. Kandal ER, Schwartz JH, Jessell TM, eds.© McGraw-Hill Companies, Inc. USA. 2000; p936–947.
7. Bae CJ, Foldvary-Schaefer N. Normal human sleep. In: Carney PR, Berry RB, Geyer JD, eds. Clinical Sleep Disorders.© Lippincott Williams & Wilkins, Philadelphia, USA, 2005: 29–37.