Doubtless, a massive crush on Jon Stewart or a labor-intensive hairstyle factors into when you go to sleep and when you wake up. But there's a stronger force at work: We're genetically predisposed to follow the commands of a unique body clock, aka circadian rhythms, which subconsciously influence our sleeping habits, as well as eating patterns, heart rate, body temperature, hormone production and sex drive.
While most of us work and play in concert with these instinctual rhythms, we often overlook the role that chronobiology — the science of body time — plays in our love life. Until, of course, there's a problem.
"Frankly, couples don't pay enough attention to these differences," says Dr. Michael Smolensky, professor at the University of Texas School of Public Health in Houston and author of "The Body Clock Guide to Better Health." "Your chronotype is a biological trait, like having blue or brown eyes. But rhythms that are out of sync between couples can lead to discord."
Like the boyfriend who wants company on his cherished morning runs in the park, and assumes you're lazy and selfish for sleeping in — when really your natural circadian rhythm is the opposite of his.
Dr. Roberto Refinetti, editor of the Journal of Circadian Rhythms, says you can try to alter your clock, but you can't fight it. "Trying to change it is usually not practical for most people. But they can learn to negotiate it in the relationship."
One compromise is to commit to a sex schedule. If that sounds appallingly unromantic, try lying in bed, sexually frustrated at midnight when your early-bird amour has crashed before you've undone your top button, or tussling with a lover who craves slow morning sex when all you want is to hit the gym before work.
Smolensky says about one in 10 people is an early bird, while two in 10 are night owls. The rest of us fall somewhere in between.
"In couples where one is an early bird and one is the other type, they know it," says Dr. Charmane Eastman, director of the Biological Rhythms Research Lab and professor of psychology at Rush University Medical Center in Chicago. "But sometimes the rhythm differences actually work because it means that each person gets to have time alone."
If you decide to try to reset your body clock, you'll need to be dedicated, she says. Night owls trying to morph into early risers should seek exposure to light earlier in the morning. Since the most intense light is outside, try waking up at the same time each day and getting outdoors as soon as possible.
Shifting early birds should avoid bright light in the morning. Aim for darkroom-level bedroom light, stay inside as long as possible, and wear dark sunglasses when finally venturing outdoors.
But here's the kicker: Even after a couple of weeks on a new sleep schedule, a one-day lapse will shift the clock right back to its natural state.
"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.