You may believe you can function on less sleep than you need, catch up on the weekends or compensate by drinking more coffee. If only it was so simple. Here are five common beliefs about sleep and why they aren't really so:

Sleep is just a way to let your brain rest. 

People often think the brain is resting when they sleep, but it is actually more active at night than during the day, says Jim Maas, author of Sleep for Success: Everything You Must Know About Sleep But Are Too Tired to Ask, (AuthorHouse, 2010) and CEO of Sleep for Success, a consulting business based in Fort Worth, Texas. During sleep, your cardiovascular system and brain are doing a lot of work when it comes to creativity, critical thinking and memory. For example, short-term memories get registered and stored in the brain during sleep. "There's a physical change in the brain that happens only as a product of adequate sleep," Maas says.

I can get used to sleeping less.

If you believe you can condition yourself to operate on less sleep, you're wrong. When you are chronically sleep deprived, your mental performance declines, says Phil Gehrman, assistant professor of clinical psychology at the University of Pennsylvania and a member of the Penn Sleep Center. "We lose the ability to accurately judge how impaired we are." A 2003 study by researchers at the University of Pennsylvania and Harvard Medical School found that chronically reducing sleep time to six hours or less per night hurt cognitive performance as much as staying awake for as many as two nights straight. "You're going to lose the ability to focus; you have a greater likelihood of making mistakes; and you'll have greater risk taking behavior," says Michael Breus, a Scottsdale, Ariz.-based sleep specialist and author of The Sleep Doctor's Diet Plan (Rodale 2011).

I can catch up on the weekends. 

Recent studies show that if you don't get enough sleep during the workweek, sleeping in on the weekend won't easily make up for it. If you lost about two hours of sleep for five nights straight, Gehrman says, you would need to tack an extra 10 hours onto two full nights of sleep. And that's highly unlikely. Without that much extra sleep on the weekend, you will start the next week just as depleted as you were at the end of the previous week, he says. What's more, even if you did catch up on your sleep on the weekend, it won't undo the damage done in terms of lost productivity. 

Coffee is a substitute for lack of sleep.

There's no doubt that caffeine is a potent antidote for drowsiness. It inhibits adenosine, the chemical in the brain that makes us feel sleepy, but it can only go so far. Coffee might help you feel more awake, but your body doesn't get the same nourishment from caffeine that it gets from sleep. This means your thinking speed and ability to move through problems and situations will still be impaired, says Breus. "It keeps you awake and moving around, but it doesn't replace the need for sleep," he says. "Your body doesn't heal; your memory doesn't get better." What's more, besides making you jittery, excessive caffeine can also cause you to feel even sleepier than before when it starts to wear off, Gehrman says.

Sleeping longer will make me gain weight.

You might think being in bed for longer will make you less active and cause weight gain, but the opposite is true. A 2011 University of Chicago study found that lack of sleep affects metabolism and can lead to obesity. Ghrelin and leptin, the hormones in your brain that cause you to feel hungry, actually increase in your body with less sleep, Maas says. And when we are tired and sleep deprived, we tend to have cravings for high-fat, high-carbohydrate foods, Gehrman says.