Dim light at night induces depression-like behaviors during the postpartum period through circadian rhythm related pathways in mice.

Growing evidence suggests that dim light at night (dLAN) may disrupt circadian rhythms and provoke symptoms of anxiety and depression. Due to the inconvenience of pregnancy and caring for infants, there is a high prevalence of dLAN exposure among pregnant and postpartum women. However, the role and circadian mechanism of dLAN on depression, and anxiety during the postpartum period remain unclear. Pregnant mice were housed in either a light-dark cycle (LD; 12 h of 200 lux:12 h of 0 lux) or a light-dLAN cycle (dLAN; 12 h of 200 lux:12 h of 5 lux) during the gestational and postpartum periods. Depression- and anxiety-related symptoms were assessed by the open field test, sucrose preference test, and forced swim test. Hippocampal transcript profiles were examined using multi-timepoint transcriptome analysis to assess the effects of dLAN exposure. Our findings showed that dLAN significantly increased depression-like behaviors, such as decreased sugar preference and increased immobility time, and decreased levels of brain serotonin (5-HT) and brain-derived neurotrophic factor (BDNF) during the postpartum period. In addition, dLAN reduced the amplitude of circadian rest-activity behaviors and nighttime activity levels, and these disruptions were significantly related to depression-like behaviors and low levels of 5-HT. Moreover, dLAN disrupted the expressions of hippocampal circadian genes particularly Per1 in postpartum mice. These findings reveal that dLAN induces depression-like behaviors in postpartum mice, with disruptions in circadian rest-activity rhythms and rhythmic gene expression likely mediating the adverse effects of dLAN on these behaviors.