What is sleep?
From a behavioral stand-point, sleep is a reversible behavioral state of perceptual disengagement from, and unresponsiveness to, the environment (Carskadon & Dement, 2011).
Within sleep, two broadly-defined states exist: rapid-eye movement (REM) sleep and non-REM (NREM) sleep. Sleep is normally entered through NREM sleep. NREM sleep is traditionally made up of sleep stages 1,2,3, and 4, which are defined by changes in brain activity (measured using Electroencephalograpy [EEG]) and generally reflect increasing sleep depth as one progresses through to stage 4. The brain alternates between NREM and REM sleep in what is a termed a sleep cycle.
Sleep cycles tend to last approximately 90 minutes and good sleepers generally go through 4-5 sleep cycles in a given night. The composition of sleep cycles change throughout the night, with the first third of the night tending to be characterized by greater amounts of slow-wave (deep) sleep (stages 3&4), whereas REM sleep tends to dominate in the latter third of the night, coinciding with a reduction in core-body temperature. Arousal threshold (the noise required to induce an awakening) also differs according to sleep stage. It is known, for example, that subjects are hardest to wake from slow-wave sleep, reflecting greater intensity and depth of sleep.
In NREM sleep, thought content is usually absent or, if present, fragmented but logical. In contrast, REM sleep tends to be characterized by vivid and often bizarre, illogical thought content, which is internally generated. REM sleep is the stage of sleep most often associated with dreaming, and where some regional brain areas are as metabolically active as during wakefulness. REM is also defined by phasic eye-movements and muscle atonia (paralysis). In some studies eye-movement direction during REM sleep has been shown to relate to the actual direction of gaze in the self-reported dream prior to wakening. The muscle atonia during REM prevents us from acting out our dreams; and indeed when this mechanism fails this is exactly what patients with REM-sleep behavior disorder (RBD) do.
The distribution of average time spent in specific sleep stages throughout the night is as follows:
Stage 1 (2-5%)
Stage 2 (45-55%)
Stage 3 (3-8%) [SWS]
Stage 4 (10-15%) [SWS]
REM sleep (20-25%)