The Physiology of Sleep

Sleep is a part of the daily routine in everyone, even when the "normal" sleep/wake pattern is disrupted by outside factors.
Here is used the "normal" circadian rhythm (from the Latin words circa "about" and dies "the day") of day/night activities to explore the chemistry and physiology of sleep.

In humans, the circadian cycle functions in a 24 to 25 hour cycle measured from awakening after one sleep period to awakening from the next sleep period.

The physiologic devices of circadian rhythm begin when light strikes special cells within the retina of the eye which, in turn, causes these cells to secrete a special hormone that causes an area of the brain known as the suprachiasmatic nucleus to signal the pineal body to stop secreting the hormone melatonin, which has been shown to reach its highest levels during sleep. With the day progresses, a chemical called adenosine accumulates within the brain as the level of melatonin falls.

As night falls the prohibitive effects of the retinal secretions on the pineal body are removed allows the pineal to begin secreting melatonin once again. This is the presence of higher melatonin levels within certain areas of the brain (such as the thalamus and hypothalamus) that controls the urge to sleep.

* Normal sleep is divided into non–rapid eye movement (NREM) and rapid eye movement (REM) sleep.

Stages of sleep:
stage I (light sleep), stage II, stages III and IV (deep or delta-wave sleep), and REM sleep;

NREM sleep comprises stages I-IV. Sleep is an active process those cycles at an ultradian rhythm of about 90 minutes.

Waking usually transform into NREM sleep. REM follows NREM sleep and occurs 4-5 times during a normal 8- to 9-hour sleep period.
- The first REM period of the night may be less than 10 minutes in duration, while the last may exceed 60 minutes.

For the purpose of analysis, overnight sleep has been divided into 3 equal time periods:

Sleep in the first third of the night, which comprises the highest percentage of NREM; sleep in the middle third of the night; and sleep in the last third of the night, the majority of which is REM.

Consider, awakening after a full night's sleep is usually from REM sleep.