Obesity alters circadian and behavioral responses to constant light in male mice

Exposure to artificial light during the night is known to promote disruption to the biological clock, which can lead to impaired mood and metabolism. Metabolic hormone secretion is modulated by the circadian pacemaker and recent research has shown that hormones such as insulin and leptin can also directly affect behavioral outcomes and the circadian clock. In turn, obesity itself is known to modulate the circadian rhythm and alter emotionality. This study investigated the behavioral and metabolic effects of constant light exposure in two models of obesity – a leptin null mutant (OB) and diet-induced obesity via high-fat diet. For both experiments, mice were placed into either a standard Light:Dark cycle (LD) or constant light (LL) and their circadian locomotor rhythms were continuously monitored. After 10 weeks of exposure to their respective lighting conditions, all mice were subjected to an open field assay to assess their explorative behaviors. Their metabolic hormone levels and inflammation levels were also measured. Behaviorally, exposure to constant light led to increased period lengthening and open field activity in the lean mice compared to both obesity models. Metabolically, LL led to increased cytokine levels and poorer metabolic outcomes in both lean and obese mice, sometimes exacerbating the metabolic issues in the obese mice, independent of weight gain. This study illustrates that LL can produce altered behavioral and physiological outcomes, even in lean mice. These results also indicate that obesity induced by different reasons can lead to shortened circadian rhythmicity and exploratory activity when exposed to chronic light.