Circadian rhythms are fundamental, evolutionary conserved regulators of neural, endocrine, behavioral, and molecular systems that shape diverse aspects of psychiatric function.Disruption to circadian rhythms is increasingly recognized as a key mechanism contributing to the risk, onset, and course of many psychiatric and neurodevelopmental disorders, and represents a scalable target for mechanistic investigation and intervention 1,2. CIRCADIAN RHYTHM REGULATIONCircadian rhythms are endogenous, approximately 24-hour oscillations in most bodily processes that persist in constant conditions and enable organisms to anticipate daily environmental changes 3.In humans, the master circadian pacemaker resides in the suprachiasmatic nucleus (SCN) of the anterior hypothalamus.The SCN coordinates most bodily rhythms, including sleep-wake behavior, hormone secretion, metabolism, and affective and cognitive processes.Because the intrinsic period of the SCN varies around 24 h, exogenous time cues (zeitgebers) play a central role in entraining the SCN to the 24-hour day.Environmental light is the dominant zeitgeber.Intrinsically photosensitive retinal ganglion cells (ipRGCs) containing the blue light-sensitive photopigment melanopsin transmit light signals via the retinohypothalamic tract directly to the SCN, synchronizing SCN neurons to the external lightdark cycle.Nonphotic zeitgebers, including physical activity, social interaction, or meals, reach the SCN through indirect pathways.The SCN integrates environmental and behavioral signals to maintain internal temporal alignment with the external day.At the cellular level, circadian timing is generated by transcription-translation feedback loops, producing ~24 h oscillations in gene expression 2.These molecular clocks are present in nearly all cells and tissues, forming peripheral oscillators that regulate local transcriptomic and metabolic programs.The SCN synchronizes peripheral clocks through neural and humoral outputs, including melatonin secretion, cortisol rhythms, body temperature cycles, autonomic signals, and behavioral rhythms.This hierarchical organization, in which cell-autonomous molecular clocks are coordinated by the SCN and entrained by environmental zeitgebers, provides a mechanistic framework for understanding how circadian disruption can propagate across brain circuits and peripheral systems implicated in psychiatric illness. EVALUATING CIRCADIAN RHYTHM FEATURESHuman circadian rhythms are characterized by three core features -Phase, Period, and Amplitude.Phase refers to the position of the
Soehner et al. (Thu,) studied this question.