REM sleep, or rapid eye movement sleep, is the stage of sleep in which most dreaming occurs.

Slow-wave sleep, also known as deep sleep, is the stage of sleep in which the body is most relaxed.

Circadian rhythm is the term for the body's natural sleep-wake cycle.

The average length of a sleep cycle is 90 minutes.

Melatonin is the hormone that helps regulate sleep.

Insomnia is the condition in which a person has difficulty falling or staying asleep.

Narcolepsy is the condition in which a person falls asleep suddenly and uncontrollably.

Sleep apnea is the condition in which a person stops breathing for short periods of time during sleep.

Restless legs syndrome is the condition in which a person has an irresistible urge to move their legs.

Lucid dreaming is the condition in which a person dreams vividly and often.

Nightmares are the condition in which a person has bad dreams.

Sleepwalking is the condition in which a person walks in their sleep.

Sleep talking is the condition in which a person talks in their sleep.

Sleep inertia is the condition in which a person has difficulty waking up in the morning.

Sleep deprivation is the condition in which a person feels tired and sluggish during the day.