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. 

• 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 non...

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.

Our molecular clock inside our cells aims to keep us in sync with the sun. 

• 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, ...

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. 

Thomas Edison said that sleep is "a bad habit." Like Edison, we seem to think of sleep as an adversary and try to fight it at every turn. The average American sleeps less than the recommended seven hours per night, mostly due to electric lights, television, computers, and smartphones. 

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.

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.