There are two basic types of sleep, linked to specific brain waves and neuronal activity: 

1. Rapid Eye Movement (REM) sleep:
   1. occurs about 90 minutes after falling asleep,
   2. eyes move rapidly from side to side behind closed eyelids,
   3. mixed-frequency brain wave activity is closer to that in wakefulness,
   4. breathing is faster and irregular,
   5. heart rate and blood pressure increase to near waking levels,
   6. arm and leg muscles are temporarily paralyzed,
   7. most of dreaming happens.
2. Non-REM sleep, which has three different stages:
   1. Stage 1, short relatively light sleep, heartbeat, breathing, and eye movements slow, muscles relax, brain waves begin to slow.
   2. Stage 2, light sleep, heartbeat and breathing slow, and muscles relax even further, body temperature drops, eye movement stops, brain wave activity slows with brief bursts of electrical activity.
   3. Stage 3 , deep sleep, occurs in longer periods during the first half of the night, heartbeat and breathing slow to their lowest levels, it may be difficult to be awaken,  brain waves become even slower.

Sleep is natural periodic state of rest for the mind and body with a decrease in movement and responsiveness to external stimuli.

Brain structures involved with sleep:

1. Hypothalamus controls centers affecting sleep and arousal, receives information about light exposure directly from the eyes and controls behavioral rhythm. 
2. Brain stem communicates with the hypothalamus to control the transitions between wake and sleep, produces a chemical GABA, which reduces the activity of arousal centers, it sends signals to relax muscles in REM sleep to prevent acting out of dreams.
3. Thalamus tunes out external information, and during REM sends sensations from the senses to the cerebral cortex, which interprets and processes information from short- to long-term memory.
4. Pineal gland increases production of the hormone melatonin, matching the circadian rhythm to the external cycle of light and darkness.
5. Basal forebrain releases adenosine.
6. Amygdala processes emotions during REM sleep.

You cycle through all stages of non-REM and REM sleep several times during a typical night, with increasingly longer, deeper REM periods.

Internal biological mechanisms:

1. Circadian rhythms direct many functions, including  timing of sleep, body temperature, metabolism, and the release of hormones; synchronize with environmental cues.
2. Sleep-wake homeostasis reminds the body to sleep and regulates sleep intensity.