Sleep is one of the body’s main survival methods. Why is this true and how does it work? Find out in the Mayo Clinic Guide to Better Sleep, where Mayo Clinic sleep experts Timothy I. Morgenthaler, M.D., and Bhanu Prakash Kolla, M.B.B.S., M.D., guide readers through everything they need to know about sleep. The book contains over 300 pages of detailed and helpful facts as shown in the following excerpt that focuses on the many functions of sleep. By reading this guide, you’ll gain extensive knowledge about how sleep works and the benefits that come from quality sleep. This can set the foundation for improving your sleep with the practical tips and advice on specific sleep problems that follow.
Vital functions of sleep
It’s easy to think of sleep simply as a period of rest when your body shuts down and takes a break. Just the opposite: Sleep encompasses one of the most productive parts of your day — an astonishingly busy stretch when your body ticks through a lengthy to-do list to help keep you healthy and mentally sharp.
After a long day interacting with the world, during sleep your body turns its attention inward. It goes to work repairing cells, regrowing tissue and arming your immune system to hunt down invaders intent on causing disease or infection. At the same time, a cleaning system in your brain gives your cells a bath, washing away harmful waste products generated during a day of mental labor. Your brain sorts through the day’s experiences, storing some as memories and tossing others as trivial and utterly forgettable. Acting something like its own therapist, your brain during sleep smooths out painful emotional edges and fortifies your resilience to face another day. Getting all this and more done each night is an elaborate production involving your brain’s internal messaging systems, composed of hormones, chemical messengers and electrical impulses.
Body restoration and hormone release
After the wear and tear of a long day, your body basically becomes a repair shop at night. Cellular division to regrow damaged tissue kicks into high gear, and healing processes peak. Stem cells, the raw materials that form different parts of your body, go to work, producing blood cells, growing hair, regrowing nerve cells and more.
At the same time, hormones — which act as internal messengers — surge through your body, triggering and implementing several important processes and functions. Growth hormone, well known for helping children achieve their full height, continues to be released during sleep throughout your life, though its levels start to decline beginning in your 30s and 40s. This hormone plays a key role in helping to maintain a healthy metabolism, control body fat and restore tissues.
Sleep is also key to the production of two hormones that work in tandem to regulate your appetite by making you feel either hungry or full. The hormone leptin, which makes you feel full and suppresses appetite, increases during sleep, allowing you to go several hours without being disturbed by hunger. At the same time, ghrelin, a hormone that makes you feel hungry, is kept in check. But poor sleep can upset this balance, driving down levels of leptin (the “I feel full” hormone) and boosting levels of ghrelin (the “I’m hungry” hormone). The imbalance can leave you feeling unsatisfied after eating, potentially contributing to nagging hunger and putting you at increased risk of obesity.
Several other hormones are released during sleep as well, including melatonin, which is synced by the light of day and the dark of night; thyroid-stimulating hormone, which governs your body’s metabolic rate; and cortisol, which helps you respond to stress.
Strengthening your disease defense
Sleep is crucial to a healthy immune system. It boosts your ability to fight off disease and infection. And when you’re sick, it allows you to heal and recover. A significant body of research has demonstrated that a good night’s sleep helps fend off common maladies, such as a cold.
Sleep also can help you heal when you do get sick, and it helps vaccines work better, too. After you receive a vaccine — say, for COVID-19 or the flu — your body requires sleep to respond fully and build a robust immune defense. If you don’t get enough sleep, your body produces fewer antibodies, the protective proteins that fight specific invaders. Producing fewer antibodies renders the vaccine less effective. Studies have shown that sleep deprivation before or after getting a vaccine can reduce by half the number of disease-fighting antibodies your body produces. So, when you receive a vaccine, prioritize getting a good night’s sleep in the days before and after the appointment.
Emptying the trash
The cells in your brain work hard as they think, remember, sense the outside world and oversee your body’s physical operations. Doing all that work consumes huge amounts of energy and creates a lot of waste. As that debris accumulates, it can cause a host of short- and long-term problems. During sleep, your brain seizes the opportunity to clear itself of debris by activating an internal power-washing system.
It wasn’t until 2012 that scientists discovered this neurological plumbing operation, which they dubbed the glymphatic system. They found that when you’re in the deepest stages of sleep, space is created between tiny blood vessels and certain brain cells, creating a network of channels through which fluid can flow to bathe each cell. This fluid carries blood sugar, called glucose, and other nutrients to help feed the brain’s enormous energy demands. Waste products produced by the brain are carried away in the same fluid current.
One such waste product is beta-amyloid, a sticky protein that can accumulate and form plaques in the brain. The protein is associated with degenerative conditions including Alzheimer’s disease and other dementias. Research in rodents and humans indicates that beta amyloid residue increases with sleep deprivation. Also, as you get older, this waste-clearing process becomes less efficient for several reasons.
Scientists have begun to identify lifestyle variables that could affect the glymphatic system’s ability to clear the brain of daily waste. Research suggests that the system may benefit from omega-3 fatty acids, found in fish oil and in some plant sources. Exercise increases the flow of the glymphatic system, while heavy alcohol use reduces its performance.
Making memories
It’s easy to forget just how enormously complex the brain’s memory system is. You must remember some things, but not everything, or your mind will be overrun with trivial details. Some things truly are best quickly forgotten, such as the color of every car you passed on the road. But other facts and situations are far from inconsequential and must be retained for future recall if you’re to survive and thrive. This task of remembering some things and forgetting others is so complicated that your brain does much of it after you’ve closed your eyes and dozed off, providing the peace and quiet needed to sort it all out.
Making memories is a two-step process. First, facts and situations get written into your brain in a place reserved for short-term storage. Then, during sleep, they get rewritten in a different part of the brain for safekeeping. Importantly, they’re put in the right context so that you can find them even many years later. This process is called memory consolidation.
While your brain is filing away memories and learning from the experiences and events of the previous day, sleep also sets the stage for you to take in and retain new facts from the next one.
Tempering emotions
At some point in your life, you’ve likely gone to bed upset, only to wake the next morning and find you aren’t nearly as worked up as you were the night before. It’s not just the passage of time that tempered those emotions; this is also one of the beneficial functions of sleep.
One of the brain’s remarkable survival strategies is to separate the emotional content of a situation from the factual content, then emphasize the factual aspect in memory, so that when you recall an emotionally charged event, the emotional punch has been watered down.
Scientists have theorized that this nightly tempering of emotions allows you to avoid living in a constant state of anxiety, perpetually reliving traumatic memories.
Sleep also reinforces the brain’s ability to be emotionally resilient. When you don’t get enough sleep, you can become moody, irritable and overly emotional. Your thinking and reactions are more likely to be negative, with positive thoughts muted. Research has shown that the two parts of the brain involved in regulating your emotions, the amygdala and the prefrontal cortex, don’t communicate well with each other if you haven’t gotten adequate sleep the night before.
Basically, sleep provides emotional support in two important ways: helping you put what happened yesterday into the proper perspective and preparing you to have a healthy response to tomorrow’s events.
An excerpt from Mayo Clinic Guide to Better Sleep by Timothy I. Morgenthaler, M.D., and Bhanu Prakash Kolla, M.B.B.S., M.D.
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