Table 1.

List of common circadian terms and their definitions

Term	Definition
Circadian	A modifier referring to the approximately 24-h nature of an event. The word ‘circadian’ is derived from the Latin roots circa (about) and diem (day).
Entrainment	The process of clock synchronization. For example, light ‘entrains’ the biological clock.
Zeitgeber	German word meaning ‘time giver’. An external cue (such as light or food) that entrains the circadian clock.
Suprachiasmatic nucleus (SCN)	A small region of the brain consisting of bilateral nuclei that coordinately function to synchronize circadian rhythms in other tissues.
Circadian clock–controlled gene (CCG)	A gene expressed in a circadian-dependent manner, usually under the control of a promoter that contains E-box, D-box or RRE elements.
Brain and muscle Arnt-like protein-1 (Bmal1)	The mammalian bHLH-PAS transcription factor that dimerizes with Clock to activate gene transcription.
CLOCK	A bHLH-PAS transcription factor that dimerizes with BMAL1 to activate promoters that contain E boxes (CACGTG).
Cryptochrome proteins (Cry)	Transcriptional repressors that dimerize with Per to inhibit Clock–Bmal1-mediated gene transcription. In plants and invertebrates, these function as light-responsive flavoproteins.
Neuronal PAS domain protein 2 (Npas2)	A transcription factor similar to Clock and highly expressed in the forebrain. Npas2 dimerizes with Bmal1 to activate gene transcription.
Period homolog proteins (Per)	PAS domain–containing proteins that dimerize with TIM (in Drosophila) or Cry (in mammals) to inhibit Clock–Bmal1-induced gene transcription.