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Many female mammals have recurring cycles of ovulation and sexual behaviors that are regulated by reproductive hormones and confer reproductive success. In addition to sexual behaviors, circadian behavioral rhythms of locomotor activity also fluctuate across the estrous cycle in rodents. Moreover, there is a bidirectional relationship between circadian rhythms and estrous cyclicity since mice with disrupted circadian rhythms also have compromised estrous cycles resulting in fewer pregnancies. In the present study, we assessed whether extending day length, which alters circadian rhythms, normalizes estrous 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 estrous cycles. Surprisingly, extending the day length by only 2 hours per day restored regular 4- or 5-day estrous cycles to Per1/2/3 KO mice. Longer days also induced consistent 4-day, rather than 5-day, estrous cycles in wild-type C57BL/6J mice. These data demonstrate that extending daytime light exposure could be used for enhancing reproductive success.