"Like going on a diet, it's hard to keep the rhythm there," Eastman warns. Sleep-deprived night owls who really want to give their partners some alert morning time also can try napping in the middle of their normal waking period, she adds.

Ideally, of course, their sweethearts would join them, but midday sex may be the rarest luxury. Research finds that younger couples most often have sex between 10 and 11 p.m., Smolensky reports. Convenience, rather than body clocks, may explain this finding, but he says that studies, including his own, do show natural patterns: For example, people have more sex in fall and winter, when males secrete more testosterone.

Even if mismatched couples find they can't—or don't want to—change, being aware of and understanding about a partner's rhythm can ease tension. "My best advice for an early bird and a night owl in love is to respect the biology of the other person," says Eastman.