Aims: Our study addresses underlying mechanisms of disruption of the circadian timing system by low-intensity artificial light at night (ALAN), which is a growing global problem, associated with serious health consequences. Methods: Rats were exposed to low-intensity (~2 lx) ALAN for 2 weeks. Using in situ hybridization, we assessed 24-h profiles of clock and clock-controlled genes in the suprachiasmatic nuclei (SCN) and other hypothalamic regions, which receive input from the master clock. Moreover, we measured daily rhythms of hormones within the main neuroendocrine axes as well as the detailed daily pattern of feeding and drinking behavior in metabolic cages. Results: ALAN strongly suppressed the molecular clockwork in the SCN, as indicated by the suppressed rhythmicity in the clock (Per1, Per2 and Nr1d1) and clock output (arginine vasopressin) genes. ALAN disturbed rhythmic Per1 expression in the paraventricular and dorsomedial hypothalamic nuclei, which convey the circadian signals from the master clock to endocrine and behavioral rhythms. Disruption of hormonal output pathways was manifested by the suppressed and phase-advanced corticosterone rhythm and lost daily variations in plasma melatonin, testosterone, and vasopressin. Importantly, ALAN altered the daily profile in food and water intake and eliminated the clock-controlled surge of drinking two hours prior to the onset of the rest period, indicating disturbed circadian control of anticipatory thirst and fluid balance during sleep. Conclusion: Our findings highlight compromised time-keeping function of the central clock and multiple circadian outputs, through which ALAN disturbs the temporal organization of physiology and behavior.

Aims: Our study addresses underlying mechanisms of disruption of the circadian timing system by low-intensity artificial light at night (ALAN), which is a growing global problem, associated with serious health consequences. Methods: Rats were exposed to low-intensity (~2 lx) ALAN for 2 weeks. Using in situ hybridization, we assessed 24-h profiles of clock and clock-controlled genes in the suprachiasmatic nuclei (SCN) and other hypothalamic regions, which receive input from the master clock. Moreover, we measured daily rhythms of hormones within the main neuroendocrine axes as well as the detailed daily pattern of feeding and drinking behavior in metabolic cages. Results: ALAN strongly suppressed the molecular clockwork in the SCN, as indicated by the suppressed rhythmicity in the clock (Per1, Per2 and Nr1d1) and clock output (arginine vasopressin) genes. ALAN disturbed rhythmic Per1 expression in the paraventricular and dorsomedial hypothalamic nuclei, which convey the circadian signals from the master clock to endocrine and behavioral rhythms. Disruption of hormonal output pathways was manifested by the suppressed and phase-advanced corticosterone rhythm and lost daily variations in plasma melatonin, testosterone, and vasopressin. Importantly, ALAN altered the daily profile in food and water intake and eliminated the clock-controlled surge of drinking two hours prior to the onset of the rest period, indicating disturbed circadian control of anticipatory thirst and fluid balance during sleep. Conclusion: Our findings highlight compromised time-keeping function of the central clock and multiple circadian outputs, through which ALAN disturbs the temporal organization of physiology and behavior.