Medical Definition of Sleep

  • Medical Author:
    William C. Shiel Jr., MD, FACP, FACR

    Dr. Shiel received a Bachelor of Science degree with honors from the University of Notre Dame. There he was involved in research in radiation biology and received the Huisking Scholarship. After graduating from St. Louis University School of Medicine, he completed his Internal Medicine residency and Rheumatology fellowship at the University of California, Irvine. He is board-certified in Internal Medicine and Rheumatology.

Last Editorial Review: 1/25/2017

Sleep: The body's rest cycle.

Sleep is triggered by a complex group of hormones that are active in the main, and that respond to cues from the body itself and the environment. About 80 percent of sleep is dreamless, and is known as non-rapid eye movement (NREM) sleep.

During NREM sleep, the breathing and heart rate are slow and regular, the blood pressure is low, and the sleeper is relatively still. NREM sleep is divided into four stages of increasing depth of sleep: Level 1 sleep is a transition period between sleep and wakefulness; Level 2 sleep features significant slowing of heartbeat and breathing, and makes up about 50 percent of all sleep; and Level 3 and 4 (Delta) sleep are marked by very slow respiration and heartbeat. Level 4 sleep leads to rapid eye movement (REM) sleep, also known as Level 5 sleep.

Dreams occur during three to five periods of REM sleep each night. REM sleep occurs at intervals of one to two hours, and is variable in length. REM sleep is characterized by irregular breathing and heart rate, and involuntary muscle jerks.

Most adults need around eight hours of sleep on a regular schedule to function well, although some require less, and others more. (It has been said that men need an hour less sleep than women.) Children, particularly teenagers, often need nine or ten hours for optimal functioning.



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Last Editorial Review: 1/25/2017