Sleeping too little or too much can affect a man’s ability to impregnate his partner, new research suggests.

The “sweet spot” appears to be 7 to 8 hours of sleep a night, said study author Lauren Wise, a professor of epidemiology at Boston University School of Public Health.

Among the 790 couples the researchers followed, “we found both short and long sleep duration — less than 6 hours or 9 or more per night — were associated with a reduced probability of pregnancy,” Wise said.

Using 8 hours of sleep as the reference point, men who slept less than 6 or more than 9 hours a night “had a 42 percent reduced probability of conception in any given month,” she added.

The main explanation is most likely hormonal, Wise said. Fertility experts know that testosterone is crucial for reproduction and the majority of daily testosterone release in men occurs during sleep, she explained. Total sleep time, in turn, has been positively linked with testosterone levels in several studies, she added.

All of the couples in the study were trying to conceive, and they had been trying for no more than six menstrual cycles. The couples answered questions about sleep patterns and whether they had trouble sleeping. Those men who had trouble sleeping more than half the time were also less likely to impregnate their partner than those who didn’t, the researchers found.

While the study found only an association between sleep and fertility, “it can’t prove cause and effect,” Wise said.

But the link held up even after she took into account both the men’s and women’s ages, their body mass index, their frequency of intercourse, and other factors known to affect fertility.

More research is needed, Wise noted.

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