Sleep is the process in which humans and other animals periodically rest, with
greater or lesser degrees of unconsciousness and decreased responsiveness to
the surrounding world. Sleep occurs cyclically, roughly every 24 hours even
though the average human inner body clock usually runs a 24.5-25.5 hour cycle.
This cycle gets reset daily (to match 24 hours) with various stimuli such as
sunlight. One of the correlates of this cycle is the level of melatonin, which
is high at times when we tend to sleep. Some people sleep twice every 24 hours
(afternoon nap, siesta).
Stages of sleep
Human sleep are usually divided into 5 stages according to electroencephalographic
REM sleep with rapid eye movements - this includes dreaming,
Stage 1 with 50% reduction in alpha waves compared to awake resting with
eyes closed. The stage is sometimes referred to as somnolence or "drowsy
sleep". It appears at sleep onset and can be associated with so-called
Stage 2 with "splindles" (12-16Hz) and "K-complexes"
Stage 3 with delta waves (1-2Hz) 20%-50% of the time.