Doubtless, a massive crush on Jon Stewart or a labor-intensive hairstyle factors into when you to 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, a.k.a. 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."
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 amor 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 ten people are early birds, while two in ten are night owls. The rest of us fall somewhere in between.