Transcriptional activator which forms a core component of the circadian clock. The circadian clock, an internal time-keeping system, regulates various physiological processes through the generation of approximately 24 hour circadian rhythms in gene expression, which are translated into rhythms in metabolism and behavior. It is derived from the Latin roots 'circa' (about) and 'diem' (day) and acts as an important regulator of a wide array of physiological functions including metabolism, sleep, body temperature, blood pressure, endocrine, immune, cardiovascular, and renal function. Consists of two major components: the central clock, residing in the suprachiasmatic nucleus (SCN) of the brain, and the peripheral clocks that are present in nearly every tissue and organ system. Both the central and peripheral clocks can be reset by environmental cues, also known as Zeitgebers (German for 'timegivers'). The predominant Zeitgeber for the central clock is light, which is sensed by retina and signals directly to the SCN. The central clock entrains the peripheral clocks through neuronal and hormonal signals, body temperature and feeding-related cues, aligning all clocks with the external light/dark cycle. Circadian rhythms allow an organism to achieve temporal homeostasis with its environment at the molecular level by regulating gene expression to create a peak of protein expression once every 24 hours to control when a particular physiological process is most active with respect to the solar day. Transcription and translation of core clock components (CLOCK, NPAS2, ARNTL/BMAL1, ARNTL2/BMAL2, PER1, PER2, PER3, CRY1 and CRY2) plays a critical role in rhythm generation, whereas delays imposed by post-translational modifications (PTMs) are important for determining the period (tau) of the rhythms (tau refers to the period of a rhythm and is the length, in time, of one complete cycle). A diurnal rhythm is synchronized with the day/night cycle, while the ultradian and infradian rhythms have a period shorter and longer than 24 hours, respectively. Disruptions in the circadian rhythms contribute to the pathology of cardiovascular diseases, cancer, metabolic syndromes and aging. A transcription/translation feedback loop (TTFL) forms the core of the molecular circadian clock mechanism. Transcription factors, CLOCK or NPAS2 and ARNTL/BMAL1 or ARNTL2/BMAL2, form the positive limb of the feedback loop, act in the form of a heterodimer and activate the transcription of core clock genes and clock-controlled genes (involved in key metabolic processes), harboring E-box elements (5'-CACGTG-3') within their promoters. The core clock genes: PER1/2/3 and CRY1/2 which are transcriptional repressors form the negative limb of the feedback loop and interact with the CLOCK|NPAS2-ARNTL/BMAL1|ARNTL2/BMAL2 heterodimer inhibiting its activity and thereby negatively regulating their own expression. This heterodimer also activates nuclear receptors NR1D1/2 and RORA/B/G, which form a second feedback loop and which activate and repress ARNTL/BMAL1 transcription, respectively. CLOCK regulates the circadian expression of AANAT in the retinal photoreceptor cells. The preferred binding motif for the CLOCK-ARNTL/BMAL1 heterodimer is 5'-CACGTGA-3', which contains a flanking Ala residue in addition to the canonical 6-nucleotide E-box sequence (By similarity). CLOCK specifically binds to the half-site 5'-CAC-3', while ARNTL binds to the half-site 5'-GTGA-3' (By similarity). Component of the circadian clock oscillator which includes the CRY proteins, CLOCK or NPAS2, ARNTL/BMAL1 or ARNTL2/BMAL2, CSNK1D and/or CSNK1E, TIMELESS and the PER proteins (By similarity). Forms a heterodimer with ARNTL/BMAL1 (PubMed:10931848). The CLOCK-ARNTL/BMAL1 heterodimer is required for E-box-dependent transactivation, for CLOCK nuclear translocation and degradation, and for phosphorylation of both CLOCK and ARNTL/BMAL1 (By similarity). Interaction with PER and CRY proteins requires translocation to the nucleus (By similarity). Interaction of the CLOCK-ARNTL/BMAL1 heterodimer with PER or CRY inhibits transcription activation (By similarity). Interacts with ARNTL2/BMAL2 (PubMed:11554928). Shuttling between the cytoplasm and the nucleus is under circadian regulation and is ARNTL/BMAL1-dependent. Expressed in the retinal photoreceptor cells (at protein level). Isoform 1 is expressed in both the retina and pineal gland. Isoform 2 is expressed mainly in the pineal gland. Expressed in a circadian rhythm manner in the retina. Levels increase over daylight hours, reaching maximum levels between ZT12 and ZT18. Expression levels continue to oscillate during constant darkness, with levels increasing during subjective day to reach maximum levels between day/night transition. In the pineal gland, shows no robust circadian rhythm with only a slight increase in expression between ZT10-18. Ubiquitinated, leading to its proteasomal degradation. O-glycosylated; contains O-GlcNAc. O-glycosylation by OGT prevents protein degradation by inhibiting ubiquitination. It also stabilizes the CLOCK-ARNTL/BMAL1 heterodimer thereby increasing CLOCK-ARNTL/BMAL1-mediated transcriptional activation of PER1/2/3 and CRY1/2. Phosphorylation is dependent on the CLOCK-ARNTL/BMAL1 heterodimer formation. Phosphorylation enhances the transcriptional activity, alters the subcellular localization and decreases the stability of the heterodimer by promoting its degradation. Sumoylation enhances its transcriptional activity and interaction with ESR1, resulting in up-regulation of ESR1 activity. Estrogen stimulates sumoylation. Desumoylation by SENP1 negatively regulates its transcriptional activity. Undergoes lysosome-mediated degradation in a time-dependent manner in the liver.