Glossary Of Sleep And Dream Related Terms

Browse through the following sleep glossary to supplement your sleep knowledge quickly and conveniently. The glossary is sorted alphabetically and contains links to corresponding articles with more in depth explorations of the subjects.

biological clock

Consists of the control and timing mechanisms for each 24 hour period. Housed in the suprachiasmatic nuclei.

clock-dependent alerting

Signals from the biological clock that tell a person how alert to be throughout the day. Generally highest alerting occurs in evenings and mornings while lowest occurs in mid-afternoon and throughout the late night, although these schedules can be advanced or delayed.


The part of the human mind that is at the level of awareness. It was believed for quite some time that sleep was the ending point of consciousness, but with more research of dreaming, especially lucid dreaming, we now know that is not true.


A vivid, complex, hallucinatory experience during sleep generally regarded as real by the dreamer. (Learn more about dreaming...)

electroencephalography (EEG)

Brain wave activity used to measure sleep patterns. (Learn more about EEG...)

electro-oculogram (EOG)

Uses the positive charge of the cornea and the negative charge of the retina to measure eye movements during sleep. (Learn more about EOG...)

free-running circadian rhythm

When a person is isolated from day and night cues, such as lightness and darkness) their circadian rhythm lengthens slightly and is known as free-running


Refers to the period of time when a person is falling asleep. Also see hypnopompic

hypnagogic imagery (or hypnagogic hallucinations)

Usually vague and disconnected images often experienced at the onset of sleep, but sometimes manifesting as much more vivid, dream-like hallucinations. (Learn more about hypnagogic imagery...)

hypnic jerks

hypnic myoclonias


Refers to the period of time when a person is waking up. Also see hypnagogic

hypnopompic imagery (or hypnopompic hallucinations)

Usually vague and disconnected images often experienced at the termination of sleep, but sometimes manifesting as much more vivid, dream-like hallucinations. (Learn more about hypnopompic imagery...)

lucid dreaming

The phenomenon of being conscious of one's dream while one is dreaming. Skilled lucid dreamers can use this consciousness to control the course and scope of their dreams. Various devices and techniques have been developed to help encourage lucid states. (Learn more about lucid dreaming...)

minimal units of sleep

People with sleep apnea and other sleep disorders often achieve sleep in very small fragments (1 or two minutes) throughout the night before awakening and falling back asleep again. These minimal units, even if they add up to the sleep need, will still leave the sleeper tired and sleep indebted the following day because they don't seem to carry enough regenerative power.

multiple sleep latency test (MSLT)

Most widely accepted and objective test to measure the strength of daytime sleep tendency. Measures the speed of falling asleep in a regulated environment at five times during the day, at two hour intervals.

extended MSLT

Evaluates sleep tendency in the evening as well as the daytime by adding two more measurements, making a total of 7 tests.

non-REM sleep

One of the two main stages of sleep. Characterized by lack of rapid eye movements and low muscle activity. There are four stages of non-REM sleep. (Learn more about non-REM sleep...)

opponent process model

The constant interaction of clock-dependant alerting and sleep homeostasis that produces the daily cycle of sleep and wakefulness. In other words, this determines why people fall asleep or are awake at any given time of day, depending on which impulse is stronger.

perceptual disengagement

One of the two essential defining characteristics of sleep. When we are asleep we have virtually no sense of the things around us and the sights, smells, and sounds that we readily perceive during the waking state are not recognized.

Psychomotor Vigilance Test (PVT)

Developed by Dr. David Dinges, this short 10 minute test measures a person's fitness to perform tasks by having them watch a screen and press a button whenever they see a flash of light.

psychophysiology of dreaming

The relationship of mental and physical activities during dreaming.

REM behavior disorder

A sleeping disorder where a person's atonia system does not function properly, leaving them unparalyzed during REM sleep and enabling them to physically act out their dreams, often in a dangerous manner. (Learn more about REM behavior disorder...)

REM sleep

Stage of sleep where dreams and rapid eye movements occur. The brain is highly active and the body is virtually paralyzed. (Learn more about REM sleep...)

REM sleep paralysis

The body's way of preventing a sleeper from acting out their dreams in REM sleep. Directions from the brain are blocked from going to the rest of the body by the atonia system. (Learn more about REM sleep paralysis...)

retrograde amnesia

The forgetting of events that happen just before falling asleep, or in the middle of the night. Caused by the body's inability to turn short-term memories into long-term memories at this time. (Learn more about retrograde amnesia...)


One of the two essential defining characteristics of sleep. As sleep becomes deeper the awakening stimulus must be increasingly stronger to wake the sleeper up. (Learn more about reversibility...)

sleep crisis

Occurs when someone is severely sleep deprived to the point where serious damage to health or safety can occur

sleep debt

Accumulative amount of sleep obtained less that a person's sleep need. A primary factor in determining sleep tendency, as one half of the opponent process model. (Learn more about sleep debt...)

sleep homeostasis

Process by which our sleep tendency goes up when we don't have enough sleep (sleep debt) and goes down after we obtain sleep.

sleep inertia

Confusion, lack of fine motor skils, and grogginess that occurs immediately following an abrubt awakening

sleep need

Amount of sleep necessary to achieve full and static alertness in the daytime. Anything less accumulates as a sleep debt.

sleep onset

When wakefulness changes to sleep, characterized by the end of enhanced alpha rhythms in the EEG. (Learn more about sleep onset...)

sleep starts

Also called hypnic myoclonias or hypnic jerks, sleep starts are contraction jerks of the muscles of a sleeper usually occurring within the first 5 minutes of sleep onset. They are often related to the sensation of falling some people have right before sleep onset. (Learn more about sleep starts...)

sleep tendency

The propensity to fall asleep at any given time of the day. Often measured by the speed one falls asleep, the key measurement in sleep latency tests, such as the MSLT.

suprachiasmatic nuclei

Two small clusters of neurons at the base of the brain that somehow control every person's biological clock, regulating circadian rhythms.

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About This Site

Welcome! This site is continuously being created by students of Dr. William C. Dement's Sleep And Dreams course at Stanford University.

We made this site as a call to action for people all over the world to live healthier, happier, safer, and more productive lives by learning about their own sleep. We have faith that reading the information provided on this site will motivate you to be smart about your sleep deprivation and strategic about your alertness in order to live life to your fullest, most energetic potential.

In fact, we challenge you to do so! What do you say, are you up for the challenge?

The Stanford Sleep Book

Stanford Sleep Book Picture

Dr. Dement's pioneering textbook has been the core text for Sleep and Dreams since 1980, but it has just recently been made available to the wider public for the first time.

In it you'll find a more detailed account of the most important things you need to know about sleep, alertness, dreams, and sleep disorders. Studies, statistics, plus plenty of Dr. Dement's classic anecdotes painting the history of sleep medicine.

Preface | Intro | Contents | Get A Copy

More Sleep Resources

The Zeo

A revolution in personal sleep tracking, the Zeo is a wireless headband that transmits your brainwaves in realtime to a dock (pictured here) or your smartphone. The result? You can wake up and see exactly what stages of sleep you were in during the night! Unprecedented personalized sleep knowledge.

Sleep Paralysis: A Dreamer's Guide

Sleep Paralysis Treatment Book

Ever woken up paralyzed? A surprising number of us have, believe it or not. But few know the actual causes of this phenomenon, and fewer still how to exert control over it. Dream researcher and sleep paralysis expert Ryan Hurd shares breakthrough insights into how to do just that.

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