Your Personal Time Zone

You know the types: The early bird who rises with the sun, clear-headed and eager to take on the day. And the night owl, practically comatose if roused before noon, but sharp in the evening.

Whether you're a lark or an owl affects much more than the amount of coffee you need to start the day. Your circadian type affects cognitive functioning: "The part of the brain that regulates your ability to think clearly and solve problems is heavily influenced by the body clock," says David Dinges, chief of the division of sleep and chronobiology at the University of Pennsylvania. An increasing body of evidence also links one's inner clock to mood fluctuations and fitness; it's even linked to personality. Morning people tend to be introverts who are conscientious and driven, while night owls are more often impulsive extroverts. Owls also tend to score more highly on intelligence tests and are more likely to be depressed.

After waking up, early birds have higher levels of the stress hormone cortisol than do owls, which may explain their instantly chipper demeanor. Owls' heart rates are higher in the afternoon than in the morning, which may be one reason why they are sharper mentally and physically as the day goes on. Most telling, many researchers agree, are peak body temperatures, which are linked to reaction time. An early bird's temperature peaks at 3:30 P.M., while an owl's rises until 8:00 P.M.

What sets the body's clock? The determinants are neither entirely environmental nor entirely genetic. Scientists have pinpointed several genes that indicate circadian type is partially heritable.

An imprinting process after birth is also a factor: Studies show that babies born in autumn and winter tend to be early birds, while spring and summer babies often grow up to be owls. Chronobiologists believe exposure to sunlight affects one's clock for life. Summer's long days can set the clock a few hours later, leading to owlish behavior, and short winter days have the opposite effect. Another factor in the owl-versus-early-bird breakdown is age: Teens and young adults tend to be owls. But as a result of what researchers suspect is a mix of hormonal changes and cultural factors like uncompromising work schedules, most people steadily move towards early-bird-like behavior as they age.

The best way to make your inner clock work for you? Don't fight it.