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A subdiscipline of biology concerned with the timing of biological events, especially repetitive or cyclical phenomena, in individual organisms.

A term derived from the Latin phrase “circa diem,” meaning “about a day”; refers to biological variations or rhythms with a cycle of approximately 24 hours. Circadian rhythms are self-sustaining (i.e., free running), meaning that they will persist when the organism is placed in an environment devoid of time cues, such as constant light or constant darkness. For comparison, see diurnal, infradian, and ultradian.

A standardized 24-hour notation of the phase in a circadian cycle that represents an estimation of the organism’s subjective time. CT 0 indicates the beginning of a subjective day, and CT 12 is the beginning of a subjective night. For example, for a nocturnal rodent, the beginning of a subjective night (i.e., CT 12) begins with the onset of activity, whereas for a diurnal species, CT 0 would be the beginning of activity. For comparison, see Zeitgeber time.

A conventional notation for an environment kept in continuous darkness (as opposed to a light-dark cycle). For comparison, see LD.

Varying with time of day. Diurnal rhythms may persist when the organism is placed in an environment devoid of time cues, such as constant light or constant darkness. Therefore, diurnal variations can be either light driven or clock driven. For comparison, see circadian.

The process of synchronization of a timekeeping mechanism to the environment, such as to a light-dark cycle, or LD. For comparison, see free running.

The state of an organism (or rhythm) in the absence of any entraining stimuli. Typically, subjects are kept in constant dim light or constant darkness to assess their free-running rhythms. For comparison, see entrainment.

A term derived from the Latin phrase “infra diem,” meaning “less than a day”; refers to biological cycles that last more than 1 day and, therefore, have a frequency of less than one per day. For comparison, see circadian and ultradian. 

Conventional notation for a light-dark environmental cycle; the numbers of hours of light and dark are typically presented separated by a colon. For example, LD 16:8 denotes a cycle consisting of 16 hours of light and 8 hours of dark. For comparison, see DD.

The obscuring of the “true” state of a rhythm by conditions that prevent its usual expression. Usually, the phase of an entrained rhythm or the absence of entrainment (e.g., in an animal that is unable to entrain because of some defect) is said to be masked by a light cycle. For example, the aversion of a nocturnal rodent to bright light results in its activity onset appearing to coincide with the absence of light, or “lights off,” when the animal actually has been awake for hours. For comparison, see entrainment.

Sleep stages that include the “deeper” stages of sleep in which dreaming typically does not occur. Also referred to as slow-wave sleep. For comparison, see rapid eye movement sleep. 

A change in the phase of a rhythm. This change can be measured by observing a change in the timing of a phase reference point (e.g., activity onset or the nocturnal rise in the release of the hormone melatonin) from the timing expected based on previous, free-running cycles. Phase shifts may be either advances (i.e., the phase reference point occurs earlier than normal) or delays (i.e., the phase reference point occurs later than normal). 

A graphical summary of the phase shifts produced by a particular manipulation, such as a light pulse or a pharmacological treatment, as a function of the phase (i.e., circadian time) at which the manipulation occurs. Defining the PRC to light has enabled researchers to understand and predict how entrainment to light cycles is accomplished. 

A stage of light sleep characterized by rapid eye movements and associated with dreaming. Also called paradoxical sleep. For comparison, see nonrapid eye movement sleep. 

A cluster of nerve cells located in the brain region called the hypothalamus that is responsible for generating and coordinating circadian rhythmicity in mammals. 

A term derived from the Latin phrase “ultra diem,” meaning “more than a day”; refers to biological cycles that last less than 1 day and, therefore, have a frequency of more than one per day. For comparison, see circadian and infradian. 

A German word literally meaning “time-giver.” A time cue capable of entraining circadian rhythms. Light represents the most important Zeitgeber. 

A standardized 24-hour notation of the phase in an entrained circadian cycle in which ZT 0 indicates the beginning of day, or the light phase, and ZT 12 is the beginning of night, or the dark phase. For comparison, see circadian time.