During waking hours we cycle briefly through a dream-like state about every 90 minutes.

stages of sleep
During sleep we cycle a few times through five stages. Each cycle of 5 stages takes 90 to 110 minutes and gets slightly shorter as we approach waking. The number of cycles is less as we grow older.

The first cycles each night contain shorter REM stages and longer deep sleep. As the night progresses, REM stages lengthen and deep sleep shortens. By morning most time is spent in light sleep stages 1, 2, and REM.

light sleep
Stage 1
Drifting in and out of sleep and awakened easily. Eyes move slowly and muscle activity slows. People awakened often remember fragmented visual images. Many experience sudden muscle contractions similar to the "jump" we make when startled often preceded by a sensation of starting to fall.

Stage 2
Almost 50 percent of our total sleep time. Eye movements stop and brain waves become slower, with occasional bursts of rapid waves called sleep spindles.

deep sleep difficult to wake
Stage 3
Extremely slow brain waves called delta waves begin to appear, interspersed with smaller, faster waves.

Stage 4
The brain produces delta waves almost exclusively. There is no eye movement or muscle activity. People awakened often feel groggy and disoriented for several minutes. Some children experience bed-wetting, night terrors, or sleepwalking.

Activity in parts of the brain that control emotions, decision-making, and social interaction slow. Growth hormone is released in children and young adults. Many body cells increase production and reduce breakdown of proteins. This may enable repair of damage from the day’s activities.

REM sleep light sleep
Stage 5
About 20 percent of sleep time. About half of infants' sleep time.

To begin REM sleep the pons (at the base of the brain) signals the cerebral cortex through the thalamus and signals neurons in the spinal cord to disengage the limbs. If limbs do not disengage they move along with dreams. Acetylcholine which increases during waking alertness also dominates the dream state.

The first REM stage usually starts 70 to 90 minutes after falling asleep. Breathing becomes more rapid, irregular, and shallow, eyes jerk rapidly in various directions and limb muscles become temporarily inactivated. Heart rate increases and blood pressure rises. When people awaken, they often describe dreams. Almost all dreaming occurs during REM sleep.

Ability to regulate body temperature is lost so abnormally hot or cold sensations can trigger awakening.

If REM sleep is disrupted one night we often go straight into REM sleep the next night and have extended REM sleep until we catch up.

Most mammals and birds show REM sleep while reptiles and other cold-blooded animals don' t.

circadian rhythms
The suprachiasmatic nucleus (SCN) controls Circadian Rhythms. This body clock consists of a pair of pinhead-sized brain structures of about 20,000 neurons in the hypothalamus near the optic nerve crossover. In the day when the optic nerve is stimulated by light striking the retina it signals the pineal gland to switch off production of the hormone melatonin. After dark increased melatonin levels make us drowsy.

The SCN governs other sleep/wake cycle functions, including body temperature, hormone secretion, urine production, and changes in blood pressure.

Most people’s biological clocks follow a 25-hour cycle, but sunlight or bright light resets their SCN to the 24-hour cycle of the sun. Circadian rhythms can be driven by external time cues that are associated with brightness or a particular time of day like alarm clocks, morning noises, or the timing of meals.