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.