Sleep deprivation is a growing epidemic in the U.S. The National Sleep Foundation recommends that adults get 7 to 9 hours of sleep for optimal physical, mental, and cognitive health. The average American only gets 6.8 hours of sleep and 40 percent of Americans get less than seven hours. The data clearly shows that most of us don't seem to care about optimal health, but should we? Professor Michael Walker, a sleep researcher, and expert, highlights the detrimental effects of sleep in his book, "Why We Sleep." Here are 6 fascinating statistics from his book.

1.Low Testosterone

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Men who only get 5 to 6 hours of sleep per night have the testosterone level of a man that is 10 years older than them. A lack of sleep can age a man's body by a decade in terms of muscle mass and strength, sexual performance, and energy levels. A sleep deprived 20-year-old is literally trapped in the body of a 30-year-old.

2. Decreased Athletic Performance

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Researchers can measure an objective decrease in these performances when people get less than 7 hours of sleep per night. Physical training isn't the only important aspect of running a marathon. Your performance can be destroyed if you don't sleep enough the night before. Keep this in mind the next time you sign up for 5K.

3. Increased Risk of Injury

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Researchers studied a group of athletes by observing their rates of injury and their rates of sleep. They found that there was a strong and direct correlation between the two variables. This is caused by a weakening of stability muscles that are crucial for balance and strength.

4. Dreaming

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We all know the phrase "sleep on it" when we're faced with a tough decision. Research shows that a significant amount of dreaming occurs in the brain when we are in deep (REM) sleep. We often don't remember what these dreams are, but they are important for our decision-making process. While we sleep, new information is processed with old information in order to make a rational decision. Next time you have a difficult decision to make, postpone it until the morning.

5. Skill Building

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Remember when tying your shoelaces required an immense amount of focus? The same goes for when you learned how to play an instrument or drive a car. These skills eventually become automatic - we don't really even know how we do them, it just comes naturally. This is because sleep aids in automaticity by rewiring the brain and strengthening neural connections.

6. Weight Gain

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There's a hormone in our bodies called leptin which is responsible for signaling to the brain that "hey we're full - stop eating!" When we don't sleep, this hormone is suppressed and doesn't function properly. Researchers have found that people who sleep only 5 to 6 hours per day eat about 200 to 300 calories more per day. This translates to over 70,000 extra calories per year which is 20 pounds of weight!