When we fall asleep, the nearly 86 billion neurons in our brain starts to fire evenly and rhythmically. Our sensory receptors become muffled at the same time.

The first stage of shallow sleep lasts for about 5 minutes.

We spend about half our sleeping time in stage 2. It can last up to 50 minutes during the first 90-minute sleep cycle, and less during subsequent cycles. 

During this time the brain consolidates the information that has been collected during the waking hours. It makes connections you might not make otherwise. It also curates which memories to keep and which to delete.

Some scientists consider stage 3 and 4 to be one stage.  Your body rests during these stages to help cells recover. Your cells produce the most growth hormone here to mend your bones and muscles.

While all this is happening, your muscles are fully relaxed. Mental activity is limited, including dreaming. 

We can remain in stage 4 - similar to a coma or brain death - for only about 30 minutes before the brain wakes up and falls back to sleep again. 

Even healthy sleepers wake several times a night, but might not be aware of it.

Sleep is essential for maintaining a healthy immune system, body temperature and blood pressure. The lack of sleep prevents us from regulating our moods well or recovering promptly from injuries.

• More than 80 million American adults suffer from chronic sleep deprivation.
• Fatigue contributes to more than a million auto accidents each year.
• About one-third of us will suffer from some form of sleep disorder.
• People with Kleine-Levin syndrome will sleep nearly nonstop for a week or two and return to regular cycles without any side effects.
• 4 % of US adults take sleeping pills in any given month.

• The prefrontal cortex, the place in the brain that controls decision-making and problem-solving, is the first to suffer from not enough sleep.  It causes you to be more irritable, moody and irrational.
• Sleeping less than six hours a night increases the risk of depression, psychosis, and stroke.
• Lack of sleep is tied to obesity. The stomach and other organs overproduce ghrelin, the hunger hormone, which causes overeating.