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1. Introduction: Circadian Rhythms and Their Molecular Mechanisms {#sec1-ijms-18-00938} ================================================================= The function of the body is subject to different biological rhythms, the circadian one being keyed to a cycle of a 24-h day corresponding approximately to the 24-h light/dark cycle of the Earth's rotation. In fact, the range of the internal clock period for healthy adults is 24 h and 11 ± 16 min but it is set back to a 24-h cycle each day by exposure to morning light and to external clocks \[[@B1-ijms-18-00938]\]. This rhythm regulates most of our biological and behavioral functions. Its dysregulation leads to sleep disorders and major physiological disturbances. In psychiatric disorders, sleep problems (especially reduced total sleep with insomnia, longer sleep latency, nocturnal and early morning awakenings) are highly prevalent symptoms. Insomnia is the most frequent sleep problem reported in psychiatric disorders \[[@B2-ijms-18-00938]\]. In autism, a prevalence of insomnia from 50% to 80% has been reported \[[@B3-ijms-18-00938],[@B4-ijms-18-00938],[@B5-ijms-18-00938],[@B6-ijms-18-00938]\] compared to 9--50% in age-matched typically developing children \[[@B7-ijms-18-00938],[@B8-ijms-18-00938],[@B9-ijms-18-00938],[@B10-ijms-18-00938],[@B11-ijms-18-00938]\]. However, the interpretation of these results has to take into consideration the high prevalence of insomnia (30%) in typically developing individuals \[[@B12-ijms-18-00938],[@B13-ijms-18-00938],[@B14-ijms-18-00938]\]. It is noteworthy that sleep disorders are not specific to autism spectrum disorder (ASD) given that they are observed in ASD individuals with intellectual disability (ID), ID individuals without ASD, and individuals with brain injury as well as ID, suggesting a possible overlap between ASD and ID. Concerning schizophrenia, a genome-wide association study (GWAS) shows genetic correlations between sleep disorders and schizophrenia and between the late evening chronotype and risk of schizophrenia \[[@B15-ijms-18-00938]\]. No rigorous study of prevalence of sleep disorders was conducted in bipolar spectrum disorder and depression-related disorders but altered patterns of clock genes were reported in these disorders \[[@B16-ijms-18-00938],[@B17-ijms-18-00938]\]. Conversely, high rates of sleep problems and mental disorders have been reported in shift workers, including insomnia, fatigue, anxiety and depression \[[@B18-ijms-18-00938],[@B19-ijms-18-00938],[@B20-ijms-18-00938],[@B21-ijms-18-00938],[@B22-ijms-18-00938],[@B23-ijms-18-00938],[@B24-ijms-18-00938]\]. It is noteworthy that altered patterns of clock genes were also reported in shift workers \[[@B25-ijms-18-00938],[@B26-ijms-18-00938],[@B27-ijms-18-00938]\]. The objective of this article is an attempt to clarify the relationships between psychiatric disorders, altered expression of clock genes and endogenous circadian rhythm disturbances (especially altered sleep--wake rhythms). Is the disruption of circadian rhythms, and therefore of sleep--wake rhythms, a prerequisite for the genesis of psychiatric disorders? What roles do the clock genes play in the disruption of endogenous circadian rhythms? After describing the molecular mechanisms of circadian rhythms, their relationships with psychiatric disorders are discussed, focusing in particular on bipolar disorder, anxiety, depression, attention deficit disorder, schizophrenia and autism spectrum disorder (a rich literature review on this topic was available for these psychiatric disorders, which was not the case for other disorders such as addictions or dementia). 1.1. Daily and Circadian Rhythms {#sec1dot1-ijms-18-00938} -------------------------------- There are different categories of physiological rhythms with different time scales, such as ultradian, tidal, circadian, lunar, and seasonal rhythms. A daily rhythm is a regular and predictable phenomenon which is defined as a series of significant physiological changes over 24 h. The biological rhythm consists of two components: