Sleep is often the first casualty of our overscheduled lives. We’ll cut out an hour here and there in our quest to fit more into the day, working on the unspoken assumption that sleep is unproductive. “I might be wiped out tomorrow,” we think. “But if I just stay up a little later, I’ll at least have accomplished X, Y and Z.”
But to pass sleep off as an extended stretch of physiological downtime is to drastically mischaracterize it. Sleep is when our bodies are at their busiest: While our waking minds go on autopilot, some of our bodies’ most sophisticated mechanisms rev up to do the hard work involved in repairing and maintaining nearly every aspect of our physiology and psychology.
For this reason, sleep is vital for sustaining peak mental performance, stabilizing mood, bolstering immunity, coping with stress, repairing our tissues, rebalancing our biochemistry and maintaining healthy metabolism. Hundreds of biological processes occur while we snooze — all of which allow us to be more productive, alert and healthy during our waking hours.
“Sleep is a biological imperative,” says Mark Mahowald, MD, director of the Minnesota Regional Sleep Disorders Center and professor of neurology at the University of Minnesota, Twin Cities. “It is not negotiable. Any degree of sleepiness will impair performance and mood.”
Unfortunately, many Americans haven’t quite caught on. According to one poll by the National Sleep Foundation, one-third of adults complain that daytime sleepiness interferes with their lives.
Let’s be clear: Skimping on sleep will not get you ahead — not now, and not later. In fact, getting too little sleep could undermine your productivity and effectiveness, starting from the moment you begin running a deficit, and could set you up for serious health consequences well into the future.
Want to know more about sleep’s active role in maintaining your health and well-being? Consider this article your bedtime reading, and by tomorrow you’ll probably be putting sleep at the very top of your list of daily priorities.
Stages of Sleep
Sleep needs vary, but most adults require seven or eight hours a day. Uninterrupted sleep is best, because during sleep we drift through several stages, each with its own distinct role.
Stage 1: When we first nod off, we drift into light sleep. Muscle activity eases and our eyes move slowly. During this time, we can be easily awakened.
Stage 2: In the second stage of light sleep, brain waves slow. Body temperature and heart rate decrease as we prepare to enter deep sleep.
Stage 3: This is the beginning of “deep sleep,” also known as slow wave, or delta, sleep. Brain waves further slow with only occasional faster bursts. This is when the body begins to release a surge of growth hormone that helps us rebuild our damaged cells.
Stage 4: The brain produces slow delta waves almost exclusively. Muscle activity ceases. It is difficult to wake someone from this deep slumber.
REM Sleep: Rapid Eye Movement (REM) sleep is the time of dreams. REM sleep usually comes about 90 minutes after falling asleep and may last up to one hour. Breathing becomes more rapid and shallow. The eyes dart back and forth rapidly, and brain waves speed up to nearly waking levels. Heart rate and blood pressure rise, and the body loses some of its ability to regulate its temperature.
In normal human adults, about 20 to 25 percent of total sleep time is spent in REM mode, and it serves an important purpose. “We believe that REM sleep is necessary for us to feel good and energetic and refreshed,” says Alexandros N. Vgontzas, MD, director of the Sleep Research and Treatment Center at Penn State University in Hershey. “All mammals, as far as we know, have REM sleep. There are some conditions, like sleep apnea, where REM sleep is decreased. Some people believe that’s one of the reasons these people feel fatigued and sleepy.”
Sleep and the Brain
Sleep is vital in keeping us mentally sharp and alert. Neurocognitive functions, like short-term memory and high-level functions that require us to pay attention to several things at once, are particularly vulnerable to sleep loss.
“If you lose one night of sleep, your mental performance is like you’re legally drunk,” says Vgontzas. “We’ve seen this effect even in people who reduce their sleep from eight hours to six.”
Why the brain tires remains something of a mystery. Some evidence suggests that levels of the chemical adenosine in the brain play an important role. Blood levels of adenosine rise continually during waking hours, creating an urge to sleep that grows increasingly difficult to resist. During sleep, levels of adenosine decrease. Drugs like caffeine disrupt this process by blocking the adenosine receptor. Although these drugs make you more alert in the short run, they don’t erase your sleep debt.
And that deficit is cumulative: Half a night of lost sleep doesn’t just vanish but is carried forward into the next day. In one classic study, conducted at the National Institutes of Health in the 1990s, subjects placed in a dark room for 14 hours per day slept, on average, 12 hours for the first four weeks. Then, the average dropped to eight hours per night. In other words, they had to pay off their debt before they could settle into a normal sleep schedule.
Sleep and Mood
You’ve probably noticed that lack of sleep makes you short-tempered. In a meta-analysis of 19 sleep studies, psychologists Julie Pilcher, PhD, of the University of Southern Indiana in Evansville, and Allen Huffcutt, PhD, of Bradley University in Peoria, Ill., found that mood is more impaired by sleep deprivation than either cognitive or physical performance.
Often, sleep-deprived people walk around feeling grumpy without even knowing why. Some scientists hypothesize that sleep replenishes neurotransmitters such as dopamine, which facilitate various critical brain functions. When we’re deprived of sleep, nerve activity becomes dampened. As a result, we become less motivated, less quick-thinking and more vulnerable to negative moods.
“In study after study, sleep researchers have found that good sleep sets up the brain for positive feelings,” write Stanford University sleep researcher William C. Dement, MD, PhD, and Christopher Vaughan in The Promise of Sleep (Dell, 2000). “When we don’t have enough sleep, we have a sour view of circumstances: We are more easily frustrated, less happy, short tempered, less vital.”
Sleep and Stress
Sleep also plays an important role in the endocrine system, which regulates hormones in the bloodstream. It’s during sleep that our body attempts to repair the damage done by stress and prepares us to handle the new stresses coming our way.
During sleep, levels of the stress hormone cortisol decrease and we secrete more growth hormone (a key tissue-repair substance). Without enough sleep, our cortisol level can remain elevated, keeping the body in a state of alertness and driving up blood pressure, which increases the risk of heart attack and stroke.
Sleep deprivation may also lead to a rewiring of the brain’s emotional circuitry and put us into a state of hyperarousal. Researchers from Harvard and Berkeley studied 26 healthy students after either an all-nighter or a full night’s sleep. As the students looked at pictures, researchers did brain scans of the amygdala, a midbrain structure responsible for decoding emotion. The amygdala scans of the sleep-deprived participants showed 60 percent more activity than those of the participants who had slept — and more than five times the number of neurons being fired.
In participants who slept, the amygdala seemed to be talking to the medial prefrontal cortex, an outer layer of the brain that helps mediate experiences and emotions. In the sleep-deprived brains, the amygdala seemed to be rerouted to a brain stem area called the locus coeruleus, which secretes norepinephrine, a precursor of the hormone adrenaline that triggers fight-or-flight reactions.
Sleep and Weight
Sleep is essential to regulating our metabolic system. Studies have associated sleep loss with changes in appetite and disturbances in our bodies’ use of glucose, and have suggested that sleep-deprived people can become resistant to insulin.
“If you don’t sleep well, you can develop something almost like a prediabetic condition — an ineffective use of insulin,” says Vgontzas. As a result, he adds, a sleep-deprived person needs more insulin to achieve a normal level of blood sugar. This means that we wolf down more calories — and put ourselves at risk for weight gain.
University of Chicago researchers found that subjects who slept only four hours a night for two nights had a 28 percent increase in ghrelin, a hormone that triggers hunger, and an 18 percent decrease in leptin, a hormone that tells the brain that it has eaten enough. Subjects reported a 24 percent increase in appetite, with a particular craving for sweets, salty foods and starches.
Sleep loss can also contribute to obesity in a more indirect way. When we’re too tired, we don’t exercise as much. Worse, this can trigger an unhealthy cycle that ends in eating comfort foods by the light of the refrigerator door.
“Stress leads to lack of sleep,” says Vgontzas. “Lack of sleep leads to increased stress. Stress can lead to overeating. People eat to reduce anxiety.”
Sleep and the Immune System
Sleep is essential to maintaining our immune systems — and there’s more at stake than a case of the sniffles: Studies have linked insufficient or irregular sleep to increased risk for colon cancer, breast cancer, heart disease and diabetes.
During sleep, the immune system performs preventive maintenance. Blood levels of immune system molecules such as interleukin-1 and tumor necrosis factor (a potent cancer-killer) rise tenfold. They decline when we wake.
This system is undermined by lack of sleep: One study found that people who stayed up until 3 a.m. had 30 percent fewer natural killer T cells the next day.
Science has yet to fully explain the relationship between sleep and immunity, but the link is gradually becoming clearer. Immunological signaling molecules, known as cytokines, seem to play a communication role between the brain and immune system and help to regulate sleep.
Even mild sleep deprivation — a two-hour deficit — increases the concentration of inflammatory markers associated with many chronic ailments.
“You develop a condition of low-grade inflammation, and we know that low-grade inflammation is a pathway to cardiovascular problems and decreased longevity,” says Vgontzas. “Several studies show that when these markers are high, people are at higher risk for hypertension, heart attacks, strokes and decreased longevity.”
Sleep and Fitness Recovery
Sleep and immunity should concern everybody, but they are particular concerns for athletes and other highly active individuals. Intensive training can weaken the immune system, explains Shawn Youngstedt, PhD, assistant professor of exercise science at the University of South Carolina in Columbia. “If someone does not get enough sleep — say, less than six hours — we know this results in compromised immunity,” says Youngstedt. “We also know that, with very intensive training, people are more susceptible to illness. The combination of intensive training and not getting enough sleep might really predispose one to
Studies also suggest that even moderate restrictions in sleep — such as getting only four hours per night — may lead to higher heart rate and lower heart-rate variability, factors that can affect athletic performance. Higher heart rate and lower heart-rate variability also indicate strain associated with cardiovascular risk. Over time, this may lead to high blood pressure.
When the body enters the first stage of deep sleep, it releases a surge of growth hormone. This hormone stimulates protein synthesis, helps break down fat that supplies energy for tissue repair and stimulates cell division. This repair process is essential to recovering from athletic endeavors and the wear and tear of everyday life. In fact, some scientists theorize that the decline of deep sleep as we age may contribute to physical decline by depriving us of growth hormone.
Sleep and Aging
In part because it is implicated in so many essential immune, repair and stress-moderating functions, sleep also appears to be linked to longevity. Eve Van Cauter, PhD, a professor of medicine at the University of Chicago, has shown that the effects of significant sleep debt “mimic many of the hallmarks of aging.” She and her colleagues argue that sleep loss hastens age-related ailments such as diabetes, hypertension, obesity and memory decline.
Reclaim Your Rest
Hundreds of critically important and subtle biological processes occur during sleep. While scientists are just beginning to unravel these mysteries, nature has made it clear that sleep is one essential we can’t cut short without major repercussions.
“Right now we get two hours less sleep per night than our forefathers 150 years ago,” says Mahowald. “There’s no evidence that they needed more and we need less. Our cutback is driven by the erroneous attitude that sleep is negotiable — if you want to get one more thing done, you just stay up later or get up earlier in the morning.”
Experts like Mahowald insist that we should be doing just the opposite: prioritizing optimal sleep patterns to encourage peak performance, productivity and resiliency. All of which means that if you want to accomplish as much as humanly possible — or if you just want to enjoy doing the things you’re doing now — you should probably put sleep at the top of your to-do list.