When we close our eyes and rest our bodies for the night, our brains do not stop working. In ways that scientists are just beginning to understand, sleep is an important part of learning, memorization, and physical restoration that is as vital to the body as air, water, and food.

The transition to sleep is a multistep process that involves many changes to the body. A chemical, adenosine, builds up in the blood and creates a feeling of drowsiness. Once the lights go down, the brain begins producing melatonin—a hormone that helps us doze in the dark and wake up in the presence of light. Sleep involves a cycle of four incremental stages progressing from shallow to deep, followed by a rapid eye movement (REM) stage that includes accelerated breathing, back-and-forth eye twitching, and temporary limb paralysis. REM sleep seems to produce dreams, aid in overnight memorization, and contribute to restoration and alertness the next day.

Most people need 7 to 8 hours of sleep a night to function optimally the next day. Periods of prolonged sleep deprivation can cause fatigue and impairment and raise a person’s risk of diabetes, heart disease, and depression. Lack of sleep also alters levels of appetite and satiety hormones, leading sleepy people to eat more.

Episodes of insomnia—the inability to sleep—are often triggered by stress, distraction, pain, or illness. If insomnia lasts for more than a few weeks, the body may become used to its new sleep pattern and need to be retrained to get back to a normal schedule. Over-the-counter antihistamines or prescription hypnotic drugs can be used to induce sleep, although experts agree that cognitive behavioral therapy— working with a doctor to discover what behaviors keep you awake and learn better sleep habits—is the best way to beat chronic insomnia. Other sleep disorders include sleep apnea, in which a person’s airway becomes obstructed and he or she repeatedly stops breathing at night; restless legs syndrome, a condition that keeps people awake with a constant urge to move their legs; and parasomnias, including sleepwalking and very bad nightmares.

ADDITIONAL FACTS

  1. Leonardo da Vinci (1452–1519) was said to have slept for only 15 minutes out of every 4 hours—or a total of only 1½ hours per day. Sleep researchers have since proved that this schedule is possible to follow, but only temporarily.
  2. In the first and lightest stage of sleep, many people experience sudden involuntary muscle contractions called hypnic myoclonia, or “sleep starts,” often preceded by a sensation of falling.
  3. The first REM sleep period occurs about 70 to 90 minutes after a person falls asleep, and a complete cycle takes 90 to 110 minutes, on average.
  4. Limbs become temporarily paralyzed during REM sleep, so our dreams are only in our heads. But in a rare condition called REM sleep behavior disorder, paralysis doesn’t kick in and people may act out their dreams—often with dangerous consequences, such as wrestling with their bed partner or running into walls.