Long days restore regular oestrous cyclicity in mice lacking circadian rhythms

Many female mammals have recurring cycles of ovulation and sexual behaviours that are regulated by reproductive hormones and confer reproductive success. In addition to sexual behaviours, circadian behaviour rhythms of locomotor activity also fluctuate across the oestrous cycle in rodents. Moreover, there is a bidirectional relationship between circadian rhythms and oestrous cyclicity since mice with disrupted circadian rhythms also have compromised oestrous cycles resulting in fewer pregnancies. In the present study, we assessed whether extending day length, which alters circadian rhythms, normalises oestrous cyclicity in mice. We found that Period (Per) 1/2/3 triple knockout (KO) mice, that have disabled canonical molecular circadian clocks, have markedly disrupted oestrous cycles. Surprisingly, extending the day length by only 2 hours per day restored regular 4- or 5-day oestrous cycles to Per1/2/3 KO mice. Longer days have also induced consistent 4-day, rather than 5-day, oestrous cycles in C57BL/6J mice. These data demonstrate that extending daytime light exposure could be used for enhancing reproductive success.