The science of sleep
Medical evidence suggests that for optimum health and function, the average adult should get seven to nine hours of sleep daily. But more than 60 percent of people regularly fall short of that goal. Yet it is now recognized that adequate sleep is just as important for health as diet and exercise.
HOW WE SLEEP
Indeed, we need sleep, and, in a sense, we are programmed to be sure that we get it. The body summons sleep in two ways: by boosting circulating levels of the neurotransmitter adenosine and by sending signals from the circadian clock, which controls the body’s daily rhythms. Together, these two systems establish an ideal bedtime for each of us.
Adenosine is partly the by-product of the cell’s energy expenditure. As our cells produce power to move us through the day, adenosine is released into the bloodstream and taken by receptors in the brain region that governs wakefulness (the basal forebrain). There, it acts like a dimmer switch, turning down many of the processes associated with wakefulness such as attention, memory, and reactions to physical stimuli. As brain levels of adenosine mounts, we feel drowsier. (Caffeine keeps us awake by blocking adenosine receptors in the brain.) When we sleep, our energy needs fall, and the level of circulating adenosine drops. After a good night’s sleep, the level is at its lowest, and we are most alert.
The circadian clock regulates all body functions — not just the pattern of sleeping and waking during the 24-hour cycle, but also fluctuations in body temperature, blood pressure, and levels of digestive enzymes and various hormones. Most of us experience a major “sleepiness” peak between 12 a.m. and 6 a.m. and a minor one between 2 p.m. and 4 p.m. Of course, individuals vary. Some might be ready for bed at 9 or 10 p.m. and awake at 5 a.m., while others don’t fall asleep until well after midnight and prefer sleeping until noon.
SLEEP STUDIES
Since the earliest sleep studies, encephalography (EEG) has been used to trace the brain waves of volunteers as they slumber. EEG readings have revealed several distinct phases of sleep characterized by different brain-wave patterns. The studies show that there are two general forms of sleep, distinguished by specific brain-wave activity and the presence or absence of rapid eye movements (REM). During non-REM sleep, brain waves become slower and more synchronized, and the eyes are still; during REM sleep, brain waves are faster and less organized, and the eyes scan back and forth under the lids.
We fall into non-REM sleep in four stages that represent a continuum of shallow to deep sleep. In Stage 1, characterized by relatively fast waves, we are perched on the brink of sleep and are readily aroused. By Stage 4, or slow-wave sleep, we are dead to the world; breathing has slowed considerably and blood pressure and heart rate have dropped by as much as 30 percent. The brain becomes less responsive to external stimuli, making it difficult to awaken. Although most of the body’s systems are in “sleep mode” at this stage, some are at their most productive. Early in Stage 4, for example, the pituitary releases a pulse of growth hormone that stimulates tissue growth and repair.
REM sleep sets the stage for dreams. Our eyes are scanning back and forth, but our skeletal muscles are “paralyzed,” perhaps to keep us from acting out our dreams. REM sleep also nurtures cognition and problem solving. Studies have shown that people learning a new physical task will improve their performance overnight, but only as long as they get sufficient REM sleep.
A typical night’s sleep consists of four or five REM/non-REM cycles with occasional brief episodes of wakefulness. Most Stage 4 sleep occurs during the first two to three hours of sleep. As morning approaches, REM sleep occupies an increasing share of slumber.
At age 20, we spend an average of 7.5 hours a night sleeping — with about 90 minutes each of REM and deep sleep — and we’re awake, intermittently, for about 18 minutes. By the time we’re 60, we’re only sleeping 6.2 hours a night. REM sleep has fallen to about 75 minutes; deep sleep to less than 40; and on a typical night, we’re awake for 44 minutes, on average. However, we don’t outgrow our need for sleep; it’s just harder to come by.
WHY WE NEED SLEEP
Although sleep doesn’t trump food and water in the hierarchy of physical needs, we can’t live without it. Given the ethical limits on research involving human subjects, scientists have no direct evidence on how extended sleeplessness — that is, beyond a few days — affects human beings. However, in laboratory rats deprived of sleep for long periods (one to two weeks), the results include loss of immune function and death from infections.
In a landmark study of human sleep deprivation, University of Chicago researchers followed a group of student volunteers who slept only four hours nightly for six consecutive days. The volunteers developed higher blood pressure and higher levels of the stress hormone cortisol, and they produced only half of the usual number of antibodies to a flu vaccine. The sleep-deprived students also showed signs of insulin resistance — a condition that is a precursor of type 2 diabetes and metabolic slowdown. All the changes were reversed when the students made up the hours of sleep they had lost. The Chicago research helps explain why chronic sleep debt raises the risk of obesity, heart disease, stroke, and diabetes.
Sleep loss exacts a toll on the mind as well as the body, as shown by a 2003 controlled study at the University of Pennsylvania and Harvard Medical School. The researchers studied 48 healthy men and women, ages 21 to 38, who had been averaging seven to eight hours of sleep nightly. They assigned three-quarters of the volunteers at random to three different groups that slept either eight, six, or four hours nightly; a fourth group agreed to go without sleep for three days. Every two hours during their waking periods, all the participants completed sleepiness evaluation questionnaires and took tests of reaction time, memory, and cognitive ability.
Over the course of two weeks, reaction times in the group that slept eight hours a night remained the same, and their scores on the memory and cognitive tasks rose steadily. In contrast, scores for the four-hour and six-hour sleepers drew closer to those of the fourth group, whose scores had plummeted during their three days without sleep.
HORMONES AND SLEEP
While scientists have long known that many hormones are affected by sleep, recent research into the impact of sleep on weight gain has thrown a spotlight on two chemicals in particular: leptin and ghrelin. Both can influence appetite, and studies now show that their production in the body may be influenced by how much or how little we sleep.
If you’ve ever had the experience of a sleepless night followed by a day that you never felt full or satisfied, no matter how much or what you ate, you have felt the effects of leptin and ghrelin. The two chemicals work in a check-and-balance system to control feelings of hunger and fullness. Ghrelin, which is produced in the gastrointestinal tract, stimulates appetite, while leptin, produced in fat cells, sends a signal to the brain once you are full.
When you don’t get enough sleep, it drives leptin down, which means you don’t feel as satisfied after you eat. It also causes ghrelin levels to rise, which means your appetite is stimulated, so you want more food. This combination is a perfect recipe for overeating, leading to weight gain.
Most experts now agree that if you are dieting, logging in a few extra hours of sleep a week is not a bad idea, particularly if you currently only get six hours of sleep or less a night. In addition to lessening your risk of serious illness, you’ll likely feel better and may just discover that you aren’t as hungry, or that you have lessened your craving for sugary, calorie-dense foods.
Far from being “an expandable luxury,” it turns out that a good night’s sleep may be as essential to disease prevention and maintenance of a healthy weight as is a nutritious diet or exercise.
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Selected resources: American Academy of Sleep Medicine (www.aasmnet.org); Better Sleep Council (www.bettersleep.org); American Insomnia Association (www.americaninsomniaassociation.org); National Center on Sleep Disorders Research (www.nhlbi.nih.gov/about/ncsdr); American Sleep Apnea Association (www.sleepapnea.org); and National Sleep Foundation (www.sleepfoundation.org).
