At many locations in the northern hemisphere human sperm counts are reduced during the summer. Experiments with rhesus monkeys suggest that such seasonal variation may be induced by an endogenous biological clock reset annually by changes in the length of daylight. Little is known about seasonal variation in human female reproductive function, although fecundity may be lower during the summer, as it is for men. Variation in fecundity could help explain seasonal patterns of birth in countries such as the United States, where there is a deficit of spring births. But the relationship to patterns of birth in Canada and Europe, where the birth rate is highest during late winter and spring, is unclear.

Key terms biological clock; birth; human; light; photoperiod; sperm count