What makes some of us "night owls" -- people who perk up in the evening and don't go to bed until 2 a.m. (or even later) -- while others are "larks" -- early birds who wake up bright-eyed and ready to go at the crack of dawn? The answer lies mostly in our internal body clock, which is largely determined by our genes. In addition to driving our 24-hour (or circadian) sleep-wake cycle, this clock regulates hormone levels, body temperature, blood pressure, alertness and performance ability.

The cycles themselves are controlled mainly by a region within the brain's hypothalamus known as the suprachiasmatic nucleus. This area responds to signals from the retina, specifically, the light that travels from our eyes to our brain, which is the most important factor in orienting our bodies to daytime alertness and nighttime sleep. But that's not the whole story. The "owl" and "lark" categories account for just about 30 percent of the population, explains Michael Smolensky, Ph.D., a professor of environmental physiology at the University of Texas School of Public Health in Houston and co-author of The Body Clock Guide to Better Health (Henry Holt & Co., 2001). Smolensky says the remaining 70 percent of us are "hummingbirds" -- people who can usually adapt when they need to, though it's easier or harder to do so depending on where you fall along the body-clock continuum.

Here's how to resolve your own internal-clock dilemmas so that you're energized and alert when you most need to be.

Body-clock dilemma: "Help! I over-sleep and am late to work a lot!"

Reset solutions The only clock your boss is likely to care about is the one that says you're a half-hour late. So to get yourself going earlier in the morning, you need to get enough sleep the night before -- seven to nine hours for most people. "Make it a point to never watch the 11 o'clock news," advises Timothy Monk, Ph.D., a professor of psychiatry and director of the Human Chronobiology Research Program at the University of Pittsburgh. Not only is it on too late, but the stories can overstimulate you, making it harder to fall asleep. The same goes for scary movies or stressful activities like paying your bills. Instead, try to wind down with a good book or warm bath about 90 minutes before bedtime. Avoid bright lights, caffeine and alcohol.

When morning comes, take a short walk outdoors or open all your window shades and eat breakfast in the glow of natural light. "Get as much light into your eyes as possible first thing in the morning," Monk says. This helps your brain register that it's time to be alert. Strong coffee can jump-start your day as well, he adds.

Once at work, keep your body clock in mind when scheduling tasks: Plan easier activities -- e-mail correspondence or organizational projects -- first thing in the morning whenever possible because you're not operating at your mental best. Late morning, such as 11 a.m., through the lunch hours is better for challenging tasks, such as a meeting that requires you to think on your feet.

Body-clock dilemma: "I can't keep my eyes open after lunch."

Reset solutions Built into our circadian cycle is what researchers call a "post-lunch dip," though this afternoon sleepiness tends to occur whether or not you've had a midday meal. The dip happens around 2:30 for most people, though larks may begin to nod off sooner and owls later.

Some cultures do the sensible thing at this time -- they take an afternoon siesta. If you're among those who can control their schedule (and you have a place to nap), sleep no more than 20 minutes. Anything beyond a catnap and you will enter a deeper stage of sleep that's harder to wake from, which will leave you feeling more tired and may even interfere with getting a good night's rest.

If, on the other hand, you're stuck at your desk and you're doing head bobs, take a walk, outside if possible, or socialize for a few minutes with colleagues. If you're still dragging, coffee and other caffeinated drinks can give you a boost, but don't rely on them too heavily as they can disrupt sleep later on that night. Avoid your most taxing assignments at this time -- your work may not be up to its usual quality.

Again, experts emphasize that it's imperative not to shortchange your z's at night. "Get enough sleep and you can cruise through the day," says David N. Neubauer, M.D., associate director of the Johns Hopkins Sleep Disorders Center in Baltimore.

Body-clock dilemma: "I went to Europe for a vacation, but all I wanted to do was sleep."

Reset solutions The quickest way to throw off your body clock is to cross a few time zones. The key to adapting: Remember the old "When in Rome ... " adage and eat and sleep when the locals do -- from the get-go. If it's late afternoon in Los Angeles when you arrive but evening back home, get out and soak up the sunshine to help you stay up later.

For a good night's rest, never eat a heavy meal before bedtime, go easy on the alcohol and skip anything with caffeine -- including that chocolate on your hotel pillow. Bring along earplugs and an eye mask to help cue your body that it's time to rest.

The morning after you arrive, take a walk or run, or swim laps outdoors if you can. Both the light and the exercise will help you adjust to the new time zone.

Body-clock dilemma: "I hate exercising in the morning, but that's the only time I can find to do it."

Reset solutions Late-afternoon workouts are often the most productive, because that's when body temperature is highest, strength greatest, perceived exertion lowest and muscles warmest. But you can make morning exercise work. Getting to bed earlier the night before is the No. 1 way to beat the snooze-button-repeat routine. In the morning, don't rush straight into your workout, advises Michael Deschenes, Ph.D., an associate professor of kinesiology at the College of William and Mary in Williamsburg, Va. Get dressed in your workout attire slowly and take time -- about five to 10 minutes -- to warm up; a cold muscle is weaker and more susceptible to injury, Deschenes says. You should start your workout with a slower version of what you'll be doing -- for example, five minutes of brisk walking before graduating to running speed, or slow, gentle laps of swimming before picking up your pace.

Keep in mind that morning exercise does have its own unique advantages. Research shows that people who work out in the morning are more successful at making fitness a regular habit and sleep better at night than do evening exercisers.

Body-clock dilemma: "My boyfriend is a morning person -- and I'm definitely not."

Reset solutions You want to dance till dawn; he can't keep his eyelids open past 9 p.m. You're grumpy in the morning, while he's whistling a tune. It's no wonder there's a high divorce rate between owls and larks, says Matthew Edlund, M.D., director of the Center for Circadian Medicine in Sarasota, Fla., and author of The Body Clock Advantage: Finding Your Best Time of Day to Succeed in Love, Work, Play, Exercise (Circadian Press, 2004). Owl-lark couples can have a tough time with everything from socializing to sex, simply because their sleep-wake cycles aren't in harmony.

But there is hope, Edlund says. Couples can take steps to work with their mismatched biology. If he's a lark and you're an owl, for example, he can try exercising in the evening and using bright light to help him stay up later. And to get more in sync with his schedule, you can try a morning jog or bike ride in the sunshine.

The two of you may find that some of your more alert times overlap, such as midmorning and early evening, so plan recreational and romantic activities accordingly. On a positive note, being part of an owl-lark couple has one distinct advantage: Parents can take the child-care shift that best suits their body clocks.

Body-clock dilemma: "I want to be able to party all night!"

Reset solutions Regardless of our body-clock setting, researchers believe that most of us are night owls during our teens to early 20s, owing to surges in reproductive hormones that affect the release of melatonin, a hormone that tells the body when to sleep. While many of us shift from that temporary owl phase as we move into our 30s, natural-born owls continue to burn the midnight oil throughout life. And if they're your friends, you'll undoubtedly have trouble keeping pace, particularly if you're a lark who has trouble staying up past 9 or 10 p.m.

To perk up in the evening, expose yourself to bright light by turning on all the lamps in the room, Neubauer says. Evening cardio exercise also may boost wakefulness, as can chatting on the phone with a friend. But skip the caffeinated beverages within five hours of bedtime or you risk difficulty sleeping.