The evolution of humans has determined that we sleep about one-third of our lives away. For evolution to devote that much time to sleep, there must be some important benefits.

An early theory was that the night was a dangerous time for early humans, so it was better to shelter in place and become passive. We now know that sleep is an active process that affects every organ in the body.

Benefits for the body

While you sleep, your body is restoring and recuperating. Chemical messengers are being replaced and released. Growth hormone is being used for muscle growth and tissue repair. Damage to your heart and blood vessels may be healing. Hormones that control blood sugar and hunger are replaced. While all this activity is going on, your body is also conserving energy. Your metabolism drops by about 10 percent. Your temperature and need for calories decline.

Sleep also boosts your immunity. Not only does your immune system produce and release important proteins called cytokines when you’re sleeping, but sleep also strengthens mucosal defenses to ward off cold and flu viruses. That’s one of the reasons why a viral infection makes you sleepy: Your immune system needs to power up. Studies show that animals deprived of sleep even for a few weeks will die.

Benefits for the brain

Sleep is important for the brain too. Attention. acquisition, consolidation, and recall are the building blocks of learning and memory. Acquisition refers to taking in new information or learning a new skill. It occurs while you are awake, but lack of sleep makes it harder to focus and learn new information. Consolidation refers to the reinforcement of new and previously learned material in the brain’s networks. Research suggests that memories are embedded into your brain during sleep. How this happens remains one of the great mysteries of sleep research.

Toxic brain substances are removed during sleep. These include tau—a protein that forms tangles in the brains of people with Alzheimer’s disease—and adenosine, an organic compound that is produced when brain cells are active and using energy over the day. As adenosine builds up at the end of the day, it makes you feel sleepy. (In fact, caffeine works by blocking the effect of adenosine.)

All the benefits of sleep depend on an internal, 24-hour clock called your circadian rhythm. Sleeping at the same time and for about the same amount of time each day is essential for reinforcing this mechanism. Set by hours of daylight and darkness, the clock depends on a delicate balance of chemical messengers, like the night-time hormone melatonin and the morning hormone cortisol.

Dangers of deprivation

Adults need seven to eight hours of sleep every night. The idea that some people can go with much less sleep is a myth. One of the common signs of sleep deprivation is dozing off while watching TV or sitting at work. The most obvious danger of sleep deprivation is falling asleep while driving.

When you are sleep deprived, all of the chemical messengers, hormones, and brain chemicals you need for sleep benefits go offline and fall out of balance. Short-term sleep deprivation can cause poor concentration and judgment. It may slow your reflexes and even cause constipation, but there is usually no long-term damage and we can bounce back. Long-term, or chronic sleep deprivation, however, is as damaging as chronic stress. Conditions such as insomnia or sleep apnea increase your risk for heart disease, kidney disease, high blood pressure, diabetes, stroke, obesity, and depression. When you are tired, you may eat to stay awake. You may crave the jolt of a high-sugar meal. Over time, this can lead to obesity, high blood sugar, and even diabetes. A lack of sleep can cause depression—and be the result of it. A classic symptom of depression is waking up very early in the morning and not being able to go back to sleep.

How to protect your sleep

The key to getting a good night’s rest is following good habits, called sleep hygiene. You should aspire to maintain a schedule for sleeping, waking, eating, exercising, and winding down before bedtime. 

  • Go to sleep and wake up at the same time every day.
  • Use the hour before bedtime to wind down by reading, relaxing, or taking a bath or shower.
  • Avoid bright lights, meals, exercise, alcohol, or nicotine before bedtime.
  • Avoid caffeine after 3 p.m.
  • Get daytime exercise and spend time outdoors in sunlight.
  • Keep your bedroom dark, quiet, and cool.

If you are having trouble falling asleep, good sleep hygiene should help. If it doesn’t, you may have a sleep disorder that needs additional treatment. Common sleep disorders include insomnia, sleep apnea, restless legs syndrome, and narcolepsy. Prostate disease and bladder incontinence are common causes of disrupted sleep, as are some medications.

Don’t ignore daytime sleepiness. Just like hunger is a warning sign that you need calories for energy, sleepiness is a warning that you need sleep for the health of your mind and body. Keep a sleep diary to keep track of how you sleep and how tired you are during the day. Note if you wake up in the middle of the night or early in the morning and can’t get back to sleep. Share your diary and concerns with your doctor to determine what additional steps you may need to take. In some cases, you may need to seek the expertise of a dedicated sleep specialist.

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