Fragmented day-night cycle induces period lengthening, lowered anxiety, and anhedonia in male mice.

Light exposure at night disrupts circadian-regulated biological functions, including mood. However, the consequence of fragmenting the night period and distributing it across the 24-hr period is less understood. Here we show that fragmenting an 8 -h and 6 -h night into equally distributed 2 -h periods throughout the 24-hr day results in period lengthening of the circadian rhythm in mice. Furthermore, mice exhibited less anxiety, which indicates increased risk-taking behavior, and a lack of pleasure-seeking, known as anhedonia. The successive alley and open field tests were used to assess anxiety, while the sucrose preference test was used to assess anhedonia. Analysis of depressive-like behaviors with the forced swim and tail suspension tests were not observed. After two weeks in 12 h light - 12 h dark, mice exposed to the fragmented night recovered and exhibited normal behaviors for both anxiety and anhedonia. Our results are congruent with published studies that describe the detrimental effects of constant light conditions on circadian rhythms and mood. These findings unveil the negative impact that fragmenting the day-night cycle has on circadian rhythms and mood.