That's enough to impair your memory, your thinking ability, and your safety on the job and on the road. One Australian study found that volunteers who stayed awake just 6 hours past their normal bedtime for a single day performed as poorly on tests gauging attentiveness and reaction time as those who were legally drunk. The National Sleep Foundation's 2009 poll showed that as many as 1.9 million drivers have had a car crash or a near miss due to drowsiness in the past year.

What's worse, sleep deprivation also impairs your ability to recognize that you're not running on all cylinders. In other words, you really shouldn't be operating heavy machinery (or much else), but you don't realize it. "The ability to judge how well you're doing is probably one of the first things to go when you don't get enough sleep," says Cote. "That's why you need to take preventive measures."

Energy fix: If you miss several hours of sleep one night, consider calling in sick the next day or ask if you can work from home. (That way, you won't have to drive.) If you find yourself nodding off at your desk, take a brisk walk up and down the stairs or hall. Exercise helps you snap to, in part because the accompanying rise in body temperature appears to boost alertness for a time. If possible, set aside part of your lunch hour for a nap. Remember to set an alarm, or ask a buddy to wake you.

2. Just catch up on sleep on the weekend

Fact: Unless you have insomnia, it's theoretically possible to make up for some lost sleep by dozing longer on the weekend. But it's not realistic.

With children’s birthday parties, sports practices, and all those inevitable weekend chores, chances are you won't really be able to make up for the sleep you missed, says Dement. You'll end up finishing the week in the red, with an ever-bigger sleep debt.

Energy fix: Don't regularly skimp on weekday sleep with the expectation you'll bounce back over the weekend. If you do happen to rack up an occasional sleep debt during the workweek, try to sleep later on the weekend or take a nap so you can pay at least part of it down, Dement says. Invest in a white-noise machine to help snooze through the din of lawn mowers and your kids' afternoon games in the garden.

3. It doesn't matter when you go to sleep

Fact: Night owls are nearly 3 times more likely to experience symptoms of depression than early birds, one study found—even when they got the same total amount of sleep. Experts aren't sure exactly why, but there may be an optimal time within the 24-hour clock to fall asleep and wake up, says Lisa Shives, Dr. and sleep expert. "This and other research shows that going to bed late can be bad for your mood and your overall health."

Energy fix: If you want to shift back your bedtime, start gradually: head to bed 15 to 30 minutes earlier every few days, and make sure the lights in your home are dim for about 2 hours before that time, says Shives. Then set your alarm to wake up 7 to 8 hours late