NCI Dictionary of Cancer Terms

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The NCI Dictionary of Cancer Terms features 7,847 terms related to cancer and medicine.

Browse the dictionary by selecting a letter of the alphabet or by entering a cancer-related word or phrase in the search box.

sleep stage
(sleep stayj)
One of 5 parts or stages of the sleep cycle based on the type of brain activity that occurs during the stage. During stages 1 to 4, a person will feel drowsy, fall asleep, and move into a deep, dreamless sleep. Stage 5 is called rapid eye movement (REM) sleep, and it is during this stage that dreams occur. During several hours of normal sleep, a person will go through several sleep cycles that include REM sleep and the 4 stages of non-REM sleep (light to deep sleep).