Does anyone else remember Wake-ups? Those little pink caffeine pills with a rooster graphic could keep you up far longer than any cup of coffee ever could. I spent many university all-nighters fueled by “roosters,” cramming for exams that should have been attended to much earlier. Sometimes I could barely write my answers on a test the next day because my hands were shaking from too much caffeine and too little sleep.

Now, a cup or two of coffee each day is my only caffeine buzz, and I no longer stay up all night, but I still don’t get as much sleep as I should. Few people do. The average person sleeps 60 to 90 minutes less than they did 50 years ago. Thirty-five to 40 percent of the American adult population has problems with falling asleep or with daytime sleepiness.

 [See: How To Get a Better Night’s Sleep]

“It’s a big problem,” says Lawrence Epstein, M.D., chief medical officer for the Harvard-affiliated Sleep Health Centers and author of The Harvard Medical School Guide to a Good Night’s Sleep. “People don’t think sleep is important because we can get by with less, but we’re putting our health at risk.”

NEXT: The Powers of a Good Night's Rest

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The Powers of a Good Night’s Rest
“We spend roughly a third of our lives asleep—obviously it has a crucial function,” Epstein says. Sleep plays important roles in repair, restoration, and memory; and lack of sleep hinders cognitive function. A recent study found that going for 18–20 hours without sleep had the same negative impact on performance as a blood alcohol level of 0.1 (0.08 is considered legally drunk in the United States).

Sleep deprivation has a similar effect on physical performance. Not only does it reduce energy and motivation, but lack of sleep has a huge impact on performance. A study at Stanford University monitored players on their varsity basketball team. Researchers asked the players to increase their sleep time to roughly 10 hours a night, compared with their normal night’s sleep of 6–9 hours. When they slept more, the players had faster sprint times. Their shooting accuracy also improved, with free-throw percentage increasing by 9% and three-point field-goal percentage increasing by 9.2%.

[See: Can Sleep Really Improve Your Memory?]

Studies have proven that sleep is essential to both mental and physical performance, but what’s even more interesting is that our brains never get used to sleep deprivation. “The less sleep we get at night and the longer we go without sleep the worse our performance becomes,” Epstein says. The effects of sleep deprivation are cumulative—they never plateau. However, when you ask someone if they are sleep deprived, initially they will say yes. But as they become more and more chronically sleep deprived, they will start to say they’re not sleepy. Their performance is impaired, but after a while they no longer realize the negative effect that lack of sleep is having on their minds and bodies. “We are not good judges of how we’re being affected by lack of sleep,” Epstein says.

NEXT: Sleep and Your Health

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Sleep and Your Health
Sleep is also essential for good health—both long and short term. A new study conducted by the Erasmus University Rotterdam in the Netherlands and the University of Surrey in the United Kingdom found that depriving your body of sleep takes the same toll on your immune system as stress. “Sleep helps sustain your immune system function,” says lead researcher Katrin Ackermann. Without it you are at increased risk for a host of ailments, including diabetes, high blood pressure and heart disease.

But it’s not just chronic sleep loss you have to worry about. Ever wonder why you’re more likely to get the flu after a busy session at work or a trip to Vegas with the boys? Ackermann’s study found that after just 24 hours without sleep, detrimental changes in the body were already starting to occur. Granulocytes are a type of white blood cell that plays an essential role in immunity. When we don’t get enough sleep, granulocyte counts jump during the night, indicating a direct immune response by the body.

[See: Lack of Sleep Increases Risk of Stroke]

“Granulocytes react immediately to sleep deprivation and just one night of sleep loss can affect the immune system,” Ackermann says. Future studies need to be done in order to understand the full implications of the increase in granulocyte count resulting from sleep deprivation, but increased granulocyte counts have been shown to increase the risk of cardiovascular mortality.

NEXT: Sleep, Your Weight, and Your Sex Life

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Why Less Sleep Can Equal More Weight
Research has shown that people who habitually sleep fewer than six hours per night are much more likely to gain weight, while those who sleep seven hours are less likely to do so. Why? When you sleep, your body secretes hormones that help control appetite, energy metabolism, and glucose processing. Sleeping too little (or at irregular times) upsets the balance of these hormones. A recent study in the American Journal of Clinical Nutrition found that poor sleep can increase levels of the stress hormone cortisol. Disruption of cortisol has proven to be a large factor in weight gain, particularly around the midsection. (It also increases blood pressure levels.)

Poor sleep also increases insulin secretion. After just one or two days without adequate levels of sleep, the body is no longer able to properly metabolize glucose. Insulin is a hormone that regulates glucose processing and promotes fat storage; higher levels of insulin are associated with weight gain and also increase the risk factors for diabetes considerably.

[See: Is Sleep Deprivation Making You Fat?]

Insufficient sleep is also associated with lower levels of leptin (a hormone that alerts the brain when it has had enough food) and higher levels of ghrelin (a biochemical that stimulates appetite). As a result, poor sleep may lead to food cravings even after you have eaten an adequate number of calories. When you’re tired, you will be more likely to eat unhealthy foods to help satisfy your cravings or give you a quick energy boost.

Sex and the Sleep Deprived
If you still need another reason to get your zzz’s, how about the impact it will have on your sex life? A recent study of college students found that adequate sleep increased sexual desire and physiological desires,” says Paul Loprinzi, Ph.D., an assistant professor at Bellarmine University in Louisville, KY. Adequate sleep and exercise can also increase blood flow to the penis.

NEXT: Do You Get Enough Sleep?

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How Much Sleep Do you Need to Get?
According to the National Sleep Foundation, most people need seven to nine hours of sleep a night. That being said, quality of sleep is a factor, as is genetics. “Individuals vary greatly in the amount of sleep they need,” Ackermann says. Most people need 7.5 to 8 hours a night. Some need 9 to 10, and a lucky few can get by on as little as 4 to 5 hours per night. However, Epstein warns that more people probably think they can get by on five hours a night than actually can.

Are you Getting Enough Sleep?
“If you’re waking up and are refreshed and ready for the day, I think that’s the best sign that you have had a good and sufficient night’s sleep,” Ackermann says. “If you feel tired during the day and have problems concentrating at work or school, this could be a sign you did not get enough rest.” Since people often underestimate the amount of sleep they need, Epstein suggests giving yourself a little test the next time you have some time off. “Next time you’re on vacation for a week, sleep as long as you can,” Epstein says. At first you will find that you sleep a lot—your body is playing catch-up. But by the end of the week, you’ll be waking up earlier and feeling alert. However long you sleep on the last night of your vacation is how much sleep you need.”

[See: 7 Reasons You're Not Sleeping]

Do Sleep Needs Change?
As newborns, we sleep almost all the time. As we progress through childhood, the amount of sleep we need drops off. There’s another bump during adolescence, when a lot of growth and development occurs during sleep, but after that, sleep needs pretty much stay the same. “We continue to need the same amount of sleep as we get older,” Epstein says. The only difference is that it gets harder for some people to get in a continuous block of sleep. “It can take longer to get the same amount of sleep.”

Taking Back the Night
If you have one or two nights without much sleep, it can actually take you more than a few good nights of sleep to catch up. A recent study demonstrated that following one night of sleep deprivation, the levels of granulocytes still had not returned to baseline levels after one night of recovery sleep. “Instead, an extended sleep period of 10 hours was required,” Ackermann says. This result suggests that it is not sufficient to gain merely the hours of sleep that have been missed by having a good night’s sleep. You may need some extra zzz’s. That doesn’t necessarily mean sleeping in until 10 a.m. “You don’t need to recover all your sleep in a single block,” Epstein says. If you’re having trouble sleeping at night, an afternoon siesta can help you catch up.