Circadian rhythm

A daily cycle of biological activity is based on a 24-hour period and influenced by regular variations in the environment, such as the alternation of night and day. Circadian rhythms include sleeping and waking in animals, flower closing and opening in angiosperms, and tissue growth and differentiation in fungi. See also biological clock.

Dreams

A succession of images, thoughts, or emotions passing through the mind during sleep.

Electroencephalograph (EEG)

An instrument for measuring and recording the electric activity of the brain.

Hypersomnia

The tendency to sleep excessively.

Insomnia

Inability to obtain sufficient sleep, especially when chronic; difficulty in falling or staying asleep; sleeplessness.

Melatonin

A hormone secreted by the pineal gland in inverse proportion to the amount of light received by the retina, important in the regulation of biorhythms: in amphibians, it causes a lightening of the skin.

Napping

To sleep lightly or briefly, especially during the day.

Narcolepsy

A condition characterized by frequent and uncontrollable periods of deep sleep.

Night Terrors

A sudden feeling of extreme fear that awakens a sleeping person, usually during slow-wave sleep, and is not associated with a dream or nightmare.

Nightmares

A terrifying dream in which the dreamer experiences feelings of helplessness, extreme anxiety, sorrow, etc.

Parasomnia

Parasomnias occur in a state that lies between sleep and wakefulness.

Restless Legs Syndrome

A condition that causes an uncontrollable urge to move the legs, usually because of an uncomfortable sensation.

Sleep

To take the rest afforded by a suspension of voluntary bodily functions and the natural suspension, complete or partial, of consciousness; cease being awake.

Sleep apnea

Apnea is caused by upper airway obstruction during sleep, associated with frequent awakening and often with daytime sleepiness. It occurs most often in people who are obese or who have an obstructed respiratory tract or neurological abnormalities.

Sleep deprivation

Sleep deprivation is a condition characterized by inadequate or insufficient sleep sustained over a period of time.

Sleep paralysis

Sleep paralysis is a temporary inability to move or speak that occurs directly after falling asleep or waking up.

Sleep study

Sleep studies, also called polysomnography, are painless tests that measure how well you sleep and how your body responds to sleep problems.

Snoring

To breathe during sleep with hoarse or harsh sounds caused by the vibrating of the soft palate.

Stage 1 (Non-REM) sleep

This sleep stage is when heartbeat, eye movements, brain waves, and breathing activity begin to taper down.

Stage 2 (Non-REM) sleep

Stage 2 NREM sleep sees a continued slowing of heartbeat, breathing, muscle activity, and eye movements.

Stage 3 (Non-REM) sleep

Muscle tone, pulse, and breathing rate decrease in N3 sleep as the body relaxes even further.

Stage 4 (REM Sleep)

While your brain is aroused with mental activities during REM sleep, the fourth stage of sleep, your voluntary muscles become immobilized.

Somnambulism

The act of getting up and walking around while asleep.

Wakefulness

Wakefulness is a daily recurring brain state and state of consciousness in which an individual is conscious and engages in coherent cognitive and behavioral responses to the external world.

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