Until the 1950s, most people thought of sleep as a passive, dormant part of our lives. Doctors now know that our brains are active during sleep. Sleep affects daily functioning and physical and mental health in many ways. 

During sleep, we usually pass through different phases. These stages progress in a cycle from stage one to REM (rapid eye movement) sleep. The first few stages are light stages of sleep. The last few stages of sleep are more restorative. 

Some sleep disorders never allow a person to progress beyond the light phases. That is why they do not feel rested, even after a full night of sleep.

Since different brain signals influence sleep and wakefulness, foods and medicines that change the balance of these signals affect whether we feel alert or drowsy and how well we sleep. Caffeinated drinks and some medications can cause insomnia. Heavy smokers often sleep very lightly and have reduced amounts of REM sleep. Many people with insomnia try to solve the problem with alcohol. While alcohol does help people fall into light sleep, it also robs them of the more restorative stages of sleep by keeping them in the lighter stages of sleep.

Although scientists are still trying to learn exactly why people need sleep, animal studies show that sleep is necessary for survival. Sleep appears necessary for our nervous systems to work properly. Too little sleep leaves us drowsy and unable to concentrate the next day. Researchers have found that people with chronic insomnia have a higher incidence of psychiatric problems such as anxiety and depression and are more likely to use healthcare services.

Learn about some common conditions


It’s not uncommon to have a night or two when sleep eludes you.


Left untreated, narcolepsy can cause serious health issues. Learn how we diagnose the condition and help you find relief.

Restless Leg Syndrome

Restless legs syndrome is a neurological condition that causes unpleasant sensations in the legs and sometimes arms.

Sleep Apnea

Sleep apnea is a serious, potentially life-threatening condition that is far more common than generally understood.