Table 9.1.

Terms used in circadian rhythms research

Term	Description
Amplitude	The difference between the mean of the rhythm and the peak or the trough
Circadian Time (CT)	Time as it relates to the endogenous rhythm of an organism measured under constant conditions. CT0 is defined as the onset of activity for a diurnal animal while CT12 is the onset of activity for nocturnal animals.
DD	Constant darkness
Dampening	Persistent decrease in the amplitude of a rhythm
Entrainment	When a stable phase relationship is established between an endogenous, self-sustained oscillator and an external time giver (“Zeitgeber”)
Free-running	A state when there are no capable entrainment cues; the self-sustained rhythm of the organism is observed
LD	Light cycle followed by a dark cycle; 12:12 LD-12 h of light then 12 h of dark repeating every 24 h
LL	Constant light
Masking	A change in the rhythm that does not reflect the true phase or period of the rhythm
Period	The time it takes to complete 1 cycle
Phase	The time of any point in a rhythm
Phase shift	A shift in the time of any point in a rhythm to earlier or later (phase delay, phase advance)
PRC	Phase response curve; A graphical representation of a stimuli’s temporal effect on the phase of a rhythm
Zeitgeber Time (ZT)	Time as it relates to an external Zeitgeber (“Zeit = time, geber = giver”). ZT0 refers to lights on for a diumal organism, such as humans, and ZT12 defines lights off for a nocturnal organism, such as rodents