Stage 1 – produces high amplitude theta waves (very slow brain waves), lasts 5-10 minutes
Stage 2 – the brain begins to produce bursts of rapid, rhythmic brain wave activity known as sleep spindles, lasts 20 minutes
Stage 3 – emergence of delta waves (deep, slow brain waves), transition between light and deep sleep
Stage 4 – dreaming occurs (REM sleep), paradoxical as brain and other body systems become more active but muscles become more relaxed
Sleep does not progress through these stages in sequence. Sleep begins in stage 1 and progresses into stages 2, and 3. After stage 3 sleep, stage 2 sleep is repeated before entering REM sleep. Once REM sleep is over, the body usually returns to stage 2 sleep. Sleep cycles through these stages approximately four or five times throughout the night.
On average, we enter the REM stage approximately 90 minutes after falling asleep. The first cycle of REM sleep might last only a short amount of time, but each cycle becomes longer. REM sleep can last up to an hour as sleep progresses.
Source: Cherry (2014): http://psychology.about.com/od/statesofconsciousness/a/SleepStages.htm
A typical night’s sleep comprises of a number of ultradian cycles (lasting approximately 90 minutes), and each cycle consists of a number of stages.
The cycle then goes into reverse, so we re-enter stage 3 and then stage 2, but instead of re-entering stage 1, a different kind of sleep (active sleep) appears. Pulse and respiration rate increases, as does blood pressure, and all three processes become less regular. EEGs begin to resemble those of the waking state, showing that the brain is active, supported by increases in oxygen consumption, blood flow and neural firing in many brain structures. But it’s even more difficult to wake up from this kind of sleep than deep stage 4 sleep, which is why it’s called paradoxical sleep.
Another characteristic of active sleep is the rapid eye movements under the closed lids (REM sleep). Finally, while the brain may be very active, the body isn’t. REM is characterised by muscular paralysis (especially the muscles of the arms and legs), so that all tossing and turning and other typical movements associated with sleep in fact only occur during stages 1-4 (NREM sleep).
After 15 minutes of so in REM sleep, we re-enter NREM sleep (stages 2-4), and so another ultradian cycle begins. However, with each 90-minute cycle (of which there are four or five on average per night), the duration of REM sleep increases and that of NREM sleep decreases. The first cycle normally provides the deepest sleep and the shortest REM period. As the night goes on, we spend relatively more time in REM and less in NREM sleep.In later cycles, it becomes quite common to go from REM to stage 2, and then straight back to stage 2 (bypassing stages 3 and 4). Natural waking usually occurs during a period of REM sleep.
Source: Gross, R. (2010) Psychology The Science of Mind and Behaviour, Hodder Education: London