biological clock a physiological cell mechanism that regulates one's sleep-wake cycles.
brain waves the brain's electrical activity, which changes through the various cycles of sleep.
bruxism teeth-grinding during sleep.
chronotherapy the therapeutic use of light to change sleep-wake patterns.
circadian referring to a period of 24 hours.
circadian rhythm any 24-hour cycle within the body's regulatory system, and especially the sleep- wake cycle.
conditioned insomnia a form of insomnia in which the sufferer is so afraid of his inability to fall asleep that he cannot relax enough to drift off and thus fulfills his own fear. Also known as learned insomnia.
CPAP machine continuous positive airway pressure machine. In patients who suffer from obstructive sleep apnea, a device that blows air through a mask and into the nasal passages in order to keep the airway open during sleep, thus preventing breathing cessation and disturbed sleep.
delayed sleep phase syndrome a natural sleep cycle that has become out of sync with the sleeper's desired bedtime, causing drowsiness later than normal.
delta sleep stages 3 and 4 sleep, the deepest phase of sleep, when the brain produces mostly delta waves and its electrical activity slows dramatically.
drowsiness a strong feeling of sleepiness, marked by droopy or heavy eyelids.
electroencephalogram a measurement of the brain's electrical activity. Also called an EEG.
entrain to train one's behavior to align with the body's natural cycles, for example, to avoid going to bed until one feels drowsy.
enuresis bed-wetting.
GABA gamma-aminobutyric acid. In the brain, a neurotransmitter involved in sedation, muscle relaxation, and sleep.
homicidal sleepwalking related to sleepwalking, killing or attempting to kill someone during sleep.
hypersomnia excessive sleep.
hypnagogia that which occurs between the state of sleep and wakefulness, including hallucinations, out- of-body experiences, geometrical or other meaningless imagery behind the eyes, etc.
hypnagogic of the mental state that occurs just before one falls asleep or after one begins to wake.
hypnagogic hallucination unrelated to REM or dream sleep, a hallucination that occurs just as one is falling into or out of sleep and is considered normal.
hypnic jerk caused by a normal electrical disturbance as one falls asleep, a sudden muscle jerking in a leg, arm, or jaw, as if one has been startled.
Hypnos in Greek mythology, the personification of sleep.
hypnotics a class of drugs that facilitate sleep.
insomnia trouble falling or staying asleep.
jet lag the temporary fatigue and insomnia travelers suffer when staying in a different time zone, due to a shift in their natural sleep-wake cycle.
Klein-Levin syndrome a disorder characterized by excessive sleep, up to 20 hours at a time, and uninhibited sexual behavior.
light sleep stage 1 or stage 2 sleep, characterized by a lack of dreaming, from which sleepers may be easily awakened.
light therapy the use of very bright light to set or reset one's biological clock. It is also used to help regulate moods, especially in cases of seasonal affective disorder.
lucid dreaming any dream state in which the dreamer is aware that he is dreaming and can control part of its contents. Also known as conscious dreaming.
melatonin a sleep-regulating hormone secreted by the pineal gland.
micro-arousal a partial or brief awakening from sleep, usually not remembered by the sleeper.
micro-sleep a brief dozing, usually lasting no more than a few seconds, which may occur several times a day in people who are chronically sleep-deprived.
Morpheus in Greek mythology, the god of dreams.
narcolepsy a sleep disorder in which sufferers experience excessive daytime sleepiness and may sometimes fall uncontrollably into deep sleep at any time.
natural short sleeper one who sleeps one to three hours less than average but feels fully refreshed during the day.
nightmare a frightening or horrifying dream that occurs during REM sleep and may awaken the sleeper.
night terrors suffered by some young children, a vivid hallucination occurring during a partial awakening from deep sleep, accompanied by panic and often crying. Unlike nightmares, night terrors do not occur during dream or REM sleep and are usually not remembered.
nocturnal referring to the night or occurring at night.
nocturnal sleep-related eating disorder a disorder suffered by sleepwalkers, who get up and not only walk around during sleep but also eat and then don't remember doing so.
non-24-hour sleep-wake syndrome a disorder in which one may sleep at different times, with drowsiness coming in unpredictable fashion day by day, seen most often in blind people.
NREM non-REM sleep. Sleep without dreams or the rapid eye movement that accompanies dream sleep.
obstructive sleep apnea a cessation of breathing during sleep, caused by the closing off of part or all of the airway, triggering awakenings.
parasomnia any disorder that occurs during sleep and disturbs it.
periodic limb movement disorder also known as nocturnal myoclonus, the involuntary movement of one or more of the limbs while sleeping.
phase advance getting out of bed earlier in the morning in order to feel drowsy and fall asleep earlier in the evening.
phase delay sleeping later in the morning and consequently feeling drowsy later in the evening than normal.
Pickwickian syndrome obstructive sleep apnea that results in deficient air intake throughout the day and night, seen in obese patients. Also known as obesity- hypoventilation syndrome.
polysomnogram a physiological recording of a sleeper's breathing, brain waves, muscle movements, etc., to measure for disturbances and overall sleep quality.
rebound insomnia after cessation of sleep medications, a return or worsening of insomnia.
REM sleep a cycle of sleep that recurs throughout the night and is characterized by dreaming and by rapid eye movement under the eyelids.
REM sleep behavior disorder a disorder in which one gets out of bed, experiences hallucinations, and may physically act out a dream as it is happening, often seen as evidence of brain damage or brain tumors.
restless legs syndrome a sleep disorder characterized by involuntary jerking of the legs or a sensation of bugs crawling underneath the skin of one's legs, requiring movement of the legs for relief.
serotonin a neurotransmitter involved in moods, memory, and sleep. A shortage may cause problems with insomnia.
sexsomnia related to sleepwalking, the performing of sexual acts during sleep.
sleep apnea cessation of breathing during sleep by all causes, including obstructions in the airway and by brain abnormalities that allow the sleeper to simply stop taking breaths.
sleep architecture the cycles of sleep and their durations as they occur throughout the night and as they change over time with age, and also as graphed out on a chart.