Each sleep cycle is approximately 90 minutes in length and most people experience 5-6 cycles per night. Stages 1 to 4 are slow wave sleep (SWS) and stage 5 is rapid eye movement sleep (REM).
· STAGE ONE
o Muscles are very active and we are still alert.
o When entering a hypnogogic state, people often experience a jumping sensation.
o In this stage, you are easily woken.
o Lasts for approximately 15 minutes.
· STAGE TWO
o Muscles become less active and you are more relaxed.
o Brief bursts of activity.
o Harder to be woken, but still possible.
o Lasts for approximately 20 minutes.
· STAGE THREE
o Slight activity and difficult to wake.
o Lasts for about 15 minutes.
· STAGE FOUR
o Eye movement starts to speed up.
o Difficult to wake.
o Lasts for around 30 minutes.
o Muscles twitch but barely active.
o Brain activity increases and rapid eye movement begins.
o Bodily processes, such as heart rate and temperature, drop but pick up near the end of the stage.
o Brain waves are desynchronised and you enter a paradoxical state, where your muscles are paralysed.
o Babies sleep for about 16 hours a day but not continuously, they wake up every hour or so and their cycle is shorter than an adults.
o They experience two types of sleep:
§ Active (REM) sleep
§ Quiet (SWS) sleep