A term used to signify a relationship to the rhythmic reoccurrence of biological events approximately every 24 hours. It comes from Latin circa and dies, meaning “about” and “a day”.

Circadian clock
Biological structure that is responsible for endogenously (internally) generating observable circadian rhythms. Also referred to as: circadian pacemaker, biological clock, circadian oscillator. There is a principal biological clock located in the center of the brain in the suprachiasmatic nuclei of the hypothalamus and peripheral clocks found in tissues including the liver, the heart, the skin and oral mucosa cells, and blood cells.

Circadian period
Duration of one circadian cycle. Described as the interval between the times when a circadian rhythm achieves a given state. It therefore represents the duration of the internal biological "day".

Circadian phase
The time at which a circadian rhythm will be in a given state. The time of maximum or minimum of a given rhythm is often used to measure circadian phase. The phase of a circadian rhythm is therefore used to determine the internal "time" of the biological clock.

Circadian rhythm
Cyclic biological event or function that is endogenously (internally) generated, that reoccurs approximately every 24 hours and is persistent even when there is no time cues (social and/or environmental). 

Constant routine
A laboratory procedure used to limit the “masking” effect of everyday activities on circadian rhythms being measured. Generally the participant remains awake for 24 to 72 hours in a dimly lit environment. During this period, participants maintain very low activity levels.

The biological clock has its own “day” (on average a little more than 24 hours in humans). The entrainment is the physiological process by which the circadian clock is synchronized to the environment (i.e. to the 24 hour light-dark cycle on planet Earth).

Forced desynchrony
A laboratory procedure during which research participants live on day lengths that are different from 24 hours (eg. 20-hour, 28-hour, etc.). This procedure allows to "desynchronize" the internal biological clock from the sleep/wake cycle and to be able to quantify the respective influence of two factors. These factors are the circadian (i.e. related to the circadian clock) and the homeostatic (i.e. related to wake and sleep duration) processes.

The structural and functional unit of the nervous system, consisting of a cell body and its processes.

Phase advance
Displacement of the circadian phase, for a given rhythm, to an earlier time. For example, advance of the core body temperature minimum or the plasma melatonin maximum. This displacement is comparable to an eastward travel across time zones.

Phase delay
Displacement of the circadian phase, for a given rhythm, to a later time. For example, delay of the core body temperature minimum or the plasma melatonin maximum. This displacement is comparable to a westward travel across time zones.

All techniques for observing and recording various physiological activities necessary to determine the sleep stages. These techniques include measuring the electrical activity of the brain via an electroencephalogram (EEG), of muscle activity via an electromyogram (EMG), and of eye movements via an electro-oculogram (EOG).

Sleep/wake cycle
Alternation between sleep and wake that occurs daily. Also referred to as the rest/activity cycle.

Suprachiasmatic nuclei
Bilateral neuron clusters, located in the center of the brain, that can generate their own electrical activity in a rhythmic manner over an approximate 24-hour period. Each of the two suprachiasmatic nuclei is roughly the size of the tip of a pen.

Ultra-rapid sleep/wake cycle 
A laboratory protocol during which research participants live on a sleep/wake cycle that is much faster than the usual 24-hour cycle (e.g. 20 minutes, 120 minutes, etc.). This procedure allows to quantify the effect of the internal biological clock on various parameters (e.g., sleep, performance, heart rate, blood pressure).