Circadian rhythm

physical, mental, and behavioral changes that follow a 24-hour cycle

Dreams

a series of thoughts, images, and sensations occurring in a person's mind during sleep.

Electroencephalograph (EEG)

a machine used for electroencephalography.

Hypersomnia

medical conditions in which you repeatedly feel excessively tired during the day (called excessive daytime sleepiness) or sleep longer than usual at night

Insomnia

a common sleep disorder

Melatonin

a hormone that your brain produces in response to darkness

Napping

A midday nap can enhance alertness, mood, memory, and reduce stress

Narcolepsy

a chronic neurological disorder that affects the brain's ability to control sleep-wake cycles

Night Terrors

a sleep disorder in which a person quickly awakens from sleep in a terrified state

Nightmares

a frightening or unpleasant dream.

Parasomnia

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

a condition of body and mind that typically recurs for several hours every night, in which the eyes are closed, the postural muscles relaxed, the activity of the brain altered, and consciousness of the surroundings practically suspended.

Sleep apnea

a potentially serious sleep disorder in which breathing repeatedly stops and starts

Sleep deprivation

inadequate or insufficient sleep sustained over a period of time

Sleep paralysis

a temporary inability to move or speak that occurs directly after falling asleep or waking up

Sleep study

painless tests that measure how well you sleep and how your body responds to sleep problems

Snoring

caused by the rattling and vibration of tissues. View Source near the airway in the back of the throat

Stage 1 (Non-REM) sleep

heartbeat, eye movements, brain waves, and breathing activity begin to taper down

Stage 2 (Non-REM) sleep

a continued slowing of heartbeat, breathing, muscle activity, and eye movements.

Stage 3 (Non-REM) sleep

N3 or deep sleep,

Stage 4 (REM Sleep)

your voluntary muscles become immobilized.

Somnambulism

sleepwalking.

Wakefulness

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.