A-C | D-F | G-J | K-M | N-P | Q-S | T-Z

Declarative memory

Memory for facts (semantic memory) and for events (episodic memory); also called explicit memory.

Delayed sleep phase syndrome (DSPS)

A daily sleep/wake rhythm in which the onset of sleep and the time of awakening are later than desired. A person with DSPS wakes up later and tends to go sleep later than most individuals.

Delta activity

EEG activity of 75-microvolt amplitude (peak-to-peak) and frequency less than 4 Hz or cycles per second.


Occurring or active during the daytime rather than at night.


A series of thoughts, images, or emotions occurring during sleep and especially during REM sleep. Most dreams are not remembered, unless people wake up and immediately recall the dream.

Electroencephalogram (EEG) or electroencephalograph

A recording of brain waves obtained by attaching flat metal discs (electrodes) to the scalp. EEGs reflect the summation of the activity of millions of neurons that are close to the electrode. They show characteristic changes in brain wave amplitude and frequency during sleep and wakefulness.


Arising from within a system. For example, circadian rhythms are endogenous—they persist even when the environment and behaviors are kept constant, and occur because of influences from the circadian pacemaker in the brain.


The synchronization between two rhythms. Entrainment occurs when the internal circadian system becomes in synchrony with an external system, such as the daily light/dark cycle. This entrainment is caused by the effects of light on the internal circadian pacemaker.

Excessive daytime sleepiness (EDS)

A difficulty in staying awake during the daytime when a person intends to be alert. EDS may be marked by lapses into sleep during sedentary moments during the day.


Resulting from an external factor or change. For example, environmental stimuli such as light are exogenous.

Free running

A term that describes the circadian rhythm that is not subject to any kind of entrainment (see entry). For instance, experimental subjects in a laboratory under constant environmental conditions, including only dim light, will have circadian rhythms that “free run” at their inherent rate, normally slightly longer than 24 hours. In contrast, the circadian rhythms of people living in a normal environment with a day/night light/dark cycle are reset or entrained each day by light.

back to top

A-C | D-F | G-J | K-M | N-P | Q-S | T-Z