Table 1.

Glossary of biological rhythms terms relevant to this manuscript.

Adaptation. Change in a quality of an organism (structure or function, including behavior) that enhances its fitness.

Amplitude. The extent by which a sinusoidal oscillation deviates from its mean, defining its maximum (i.e., peak) or minimum (i.e., trough).

Biological clock or oscillator. A self-sustained oscillator-system (bridging single cells to organism) that produces rhythmic biological outputs even in the absence of external rhythmic cues, zeitgebers (see definition). Biological clocks exist across species in practically all phyla.

Biological rhythm: Rhythmic output of a biological clock or oscillator (see definition).

Chronotype. 1) proxy for phase angle of entrainment (see definition), as for example quantified by the mid-point of sleep on free days; 2) diurnal preference; and 3) sometimes subsumed under personality trait. In this review, we will only refer to 1) and 2). Used to described inter-individual differences in phenotypic expressions of circadian-regulated behavioral output.

Circadian rhythm. A biological rhythm with a period (see definition) of about 24h (derived from Latin, circa diem) that is generated by a biological clock (see definition) and that persists even in temporal isolation (i.e., in absence of all external time cues). Circadian rhythms are temperature-compensated (meaning that temperature does not change the rhythm’s period) and they can actively entrain (see definition).

Circadian system. Networks of circadian clocks within an organism. The mammalian circadian system comprises a central pacemaker (in the suprachiasmatic nucleus, SCN) and the so-called peripheral clocks in most of the cells of tissues and organs. It optimizes the daily timing of biochemical, physiological, and behavioral processes by optimizing their temporal interaction.

Diurnal. Active during the daytime, inactive during the nighttime (e.g., humans). Opposed to nocturnal (see definition).

Entrainment. Active process of synchronization of an oscillator to a zeitgeber (see definition), Entrainment will result in a stable phase relationship between the oscillator and the zeitgeber

Jet Lag. Transient misalignment between an individual’s circadian clock and the local time (e.g., the local light-dark cycle at the destination of a trans-meridian flight). The circadian system will gradually entrain to the local L/D cycle, in humans on average 1 day per hour time change.

Masking. An external cue influences rhythmic biological function, but without affecting the circadian oscillator, e.g. enhancing (positive masking) or suppressing (negative masking) effects of environmental light. Birds in a cage are a good example: they will hop from perch to perch under the control of the circadian clock, but will immediately stop if put into darkness.

Nocturnal. Active during the nighttime, inactive during the daytime (e.g., mice). Opposed to diurnal (see definition).

Period. Duration of one cycle, measured as for example the time elapsed between two peaks or troughs of a rhythm.

Phase. Timing of a cycle defined by a reference point such as its minimum or its maximum temperature, dim light melatonin onset, dawn or dusk.

Phase angle of entrainment. The difference between the phase (see definition) of a biological rhythm and and that of a zeitgeber (see definition). See also chronotype.

Range of entrainment. Range of zeitgeber periods (e.g. LD cycles) that an oscillator can stably entrain to. The experimental protocol of a ‘forced desynchrony’ specifically uses experimental light-dark cycles that are outside of the range of entrainment to distinguish investigate circadian and homeostatic regulations separately.

Zeitgeber. A rhythmic signal that circadian clocks use to actively synchronize (entrain) with the cyclic environment (normally 24 h). The light/dark (LD) cycle is the most important zeitgeber for circadian clock.