We have so many demands on our time—school, jobs, family, errands, not to mention finding some time to relax. To fit everything in, we often sacrifice sleep. But sleep affects both mental and physical health. Like exercise and a healthy diet, it’s vital to your well-being.

Of course, sleep helps you feel rested each day. But while you’re sleeping, your brain and body don’t just shut down. Internal organs and processes are hard at work throughout the night. Sleep can help you “lock in” everything you’re studying and trying to remember.

“Sleep services all aspects of our body in one way or another: molecular, energy balance, as well as intellectual function, alertness and mood,” says Dr. Merrill Mitler, a sleep expert and neuroscientist at NIH.

When you’re tired, you can’t function at your best. Sleep helps you think more clearly, have quicker reflexes, and focus better. “The fact is, when we look at well-rested people, they’re operating at a different level than people trying to get by on one or two hours less nightly sleep,” says Mitler.

“Loss of sleep impairs your higher levels of reasoning, problem-solving, and attention to detail,” Mitler explains. Tired people tend to be less productive at work and school. They’re at a much higher risk for traffic accidents. Lack of sleep also influences your mood, which can affect how you interact with others. A sleep deficit over time can even put you at greater risk for developing depression.

But sleep isn’t just essential for the brain. “Sleep affects almost every tissue in our bodies,” says Dr. Michael Twery, a sleep expert at NIH. “It affects growth and stress hormones, our immune system, appetite, breathing, blood pressure and cardiovascular health.”

Research shows that lack of sleep increases the risk for obesity, heart disease, and infections. Throughout the night, your heart rate, breathing rate and blood pressure rise and fall, a process that may be important for cardiovascular health. Your body releases hormones during sleep that help repair cells and control the body’s use of energy. These hormone changes can affect your body weight.

“Ongoing research shows a lack of sleep can produce diabetic-like conditions in otherwise healthy people,” says Mitler.

Recent studies also reveal that sleep can affect the efficiency of vaccinations. Twery described research showing that well-rested people who received the flu vaccine developed stronger protection against the illness.

A good night’s sleep consists of four to five sleep cycles. Each cycle includes periods of deep sleep and rapid eye movement (REM) sleep, when we dream. “As the night goes on, the portion of that cycle that is in REM sleep increases. It turns out that this pattern of cycling and progression is critical to the biology of sleep,” Twery says.

Sleep can be disrupted by many things. Stimulants such as caffeine or certain medications can keep you up. Distractions such as electronics—especially the light from TVs, cell phones, tablets and e-readers—can prevent you from falling asleep.

How Much Sleep Do We Need?

The amount of sleep each person needs depends on many factors, including age, and getting a full night of quality sleep is important. Infants generally require about sixteen hours a day, while teenagers need about nine hours on average. For most adults, seven to eight hours a night appears to be the best amount of sleep. The amount of sleep a person needs also increases if he or she has been deprived of sleep in previous days. Getting too little sleep creates a “sleep debt,” which is a lot like being overdrawn at a bank. Eventually, your body will demand that the debt be repaid. We don’t seem to adapt to getting less sleep than we need; while we may get used to a sleep-depriving schedule, our judgment, reaction time, and other functions are still impaired. If you’re a student, that means that sleep-deprivation may prevent you from studying, learning, and performing as well as you can.

People tend to sleep more lightly and for shorter time spans as they get older, although they generally need about the same amount of sleep as they needed in early adulthood. Experts say that if you feel drowsy during the day, even during boring activities, you haven’t had enough sleep. If you routinely fall asleep within five minutes of lying down, you probably have severe sleep deprivation, possibly even a sleep disorder. “Microsleeps,” or very brief episodes of sleep in an otherwise awake person, are another mark of sleep deprivation. In many cases, people are not aware that they are experiencing microsleeps. The widespread practice of “burning the candle at both ends” in western industrialized societies has created so much sleep deprivation that what is really abnormal sleepiness is now almost the norm.

Many studies make it clear that sleep deprivation is dangerous. Sleep-deprived people who are tested by using a driving simulator or by performing a hand-eye coordination task perform as badly as or worse than those who are intoxicated. Sleep deprivation also magnifies alcohol’s effects on the body, so a fatigued person who drinks will become much more impaired than someone who is well rested. Driver fatigue is responsible for an estimated 100,000 motor vehicle accidents and 1,500 deaths each year, according to the National Highway Traffic Safety Administration. Since drowsiness is the brain’s last step before falling asleep, driving while drowsy can—and often does—lead to disaster. Caffeine and other stimulants cannot overcome the effects of severe sleep deprivation. The National Sleep Foundation says that if you have trouble keeping your eyes focused, if you can’t stop yawning, or if you can’t remember driving the last few miles, you are probably too drowsy to drive safely.