Stages of sleep
There are different kinds of sleep, divided into five stages that are cycled through during an extended period of sleep, typically during one night (see Figure 8.1).
Figure 8.1 Stages of sleep
Stage 1: Transitional. This is the first stage of sleep and can last for five to 10 minutes. It is the transition between asleep and awake, and there may still be some awareness of activity in the surrounding environment. This state to and from consciousness to sleep is called hypnogogia and there can be an experience of images or mild hallucinations during this phase (Mavromatis, 1987). This is the stage that someone is in if they fall asleep at the wheel of a car, and is often described as microsleep. A microsleep can last from a few seconds to a minute in time.
Stage 2: Light sleep. This stage lasts 10-20 minutes. During stage 2, breathing and heart rate slows, and there is a slight decrease in body temperature. Brain waves become slower, interspersed with occasional bursts of rapid waves called sleep spindles.
Stages 3/4: Deep sleep. These are the stages of deep sleep and tend to last about 30 minutes. there is limited muscle activity, and the brain produces slow delta waves. Someone who is woken in these stages of sleep may be very groggy and could take a few minutes to wake properly. this grogginess (or sleep inertia) can last longer (up to 20-30 minutes). However, Rosekind et al. (1995) suggest that, in an emergency, the effects of adrenaline can rapidly overcome the negative effects of sleep inertia.
Stage 5: Rapid eye movement (REM) sleep. This is the stage in which dreams occur It is characterised by rapid eye movements behind the closed eyelids. Muscles relax, heart rate and brainwaves speed up, and breathing becomes rapid and shallow. There are several periods of REM sleep in a normal night’s sleep. The first period is short (5-10 minutes), but they become longer as the night progresses (see Figure 8.2). There is some evidence suggesting that REM sleep is particularly important for storing memories.
Figure 8.2 Typical sleep cycle
Following stage 5, an individual then returns back to stage 2, and cycles through stages 3, 4, before returning to REM sleep (see Figure 8.2). Most of the slow-wave sleep occurs during the beginning of a typical night’s sleep, with more REM sleep in the latter half (except in day sleep most of the REM sleep occurs at the beginning of the sleep cycle and stages 3 and 4 in the second half of the sleep cycle). However, in a severely sleep-deprived individual, the REM sleep is pushed later in the period of sleep, with sleep at stages 3 and 4 occupying a greater proportion of the time asleep (Caldwell and Caldwell, 2003). As REM sleep occupies the period of time just prior to the end of a night’s sleep, this is why many people remember their dreams.