How to Sleep

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HOW SLEEP WORKS

Before you can sleep better (and you will sleep better), you need to understand the biology of normal sleep. Early scientists believed that sleep was little more than an inactive state. It wasn’t until brain-wave activity could be reliably monitored that we discovered that our journey through the night is fascinating and complex—and far from inactive. There is a lot going on while we’re sleeping, and scientific studies of the sleeping brain are revealing not only how the human organism functions during sleep but also how our sleep affects us while we’re awake. The mystery of sleep is no longer so mysterious.

Circadian Rhythms

Inside our heads is what’s known as the circadian or biological clock. This miniature timekeeper consists of a tiny cluster of neurons that shares one purpose: to synchronize the body’s biological rhythms with the earth’s cycles of dawn and dusk.

This clock is located in the perfect place: at the base of the brain, right behind our eyes and above the junction of our optic nerves, which transmit information to the circadian clock about the light that is coming into our eyes. (The fact that this small region has the best blood supply in the entire brain underscores its vital importance. A person who suffers a major stroke is much more likely to lose their ability to speak or move their hand than suffer damage to their circadian clock!)

The circadian clock’s internal rhythm runs slightly longer than twenty-four hours, to predict the incremental changes in the earth’s rotational tilt that affect the length of our days and nights across the seasons, so it tends to overshoot. But it resets at the first burst of light to hit our eyes when we wake in the morning. Once the brain knows what time the light hits our eyes, it can predict that dawn will occur around the same time the next day. This is essential to our survival: In our ancient past, if we left the safety of the village or caves we lived in, we had to ensure we’d be back to our homes before dusk, when nocturnal predators became active. But if we left too late or returned too early, we wasted potential daylight to hunt and gather.

Our circadian clock sends us signals to begin the process of falling asleep as nighttime comes, and it emits chemicals to naturally wake us up as dawn approaches. Our goal should be to work with this clock instead of fight against it.

Because our sleep patterns are influenced by light, it’s not surprising that seasonality has an effect on our sleep needs. These changes are usually too subtle to be noticeable, but if you live nearer to the poles, like in Alaska, where the amount of light varies drastically between seasons, you may sleep more in winter and less in summer. With the prevalence of artificial light, many of us are trapped into thinking it’s always a short summer night and end up sleeping as little as possible! (This is why it’s important to control your light situation for optimal sleep wherever you are—see Night Light for more.)

You Are Most Awake Before You Go to Sleep

If sleep worked like the fuel in a car, then we would be most alert when we first wake up and become less and less alert throughout the day—just like cars’ tanks are at their maximum capacity right after we fill them with fuel and get depleted the longer we drive. But sleep and alertness work in a more complex way.

Most of us wake up somewhat sleepy, even if we’ve had a normal, restful night’s sleep. We become more alert as the morning goes on. We may begin to feel drowsy again in the afternoon (and no, the culprit is not your lunch—consider that neither breakfast nor dinner makes us very sleepy). If we can shake off our afternoon slump, in the evening we will find ourselves feeling more alert than we were earlier in the day even though our ‘“tank” is nearly empty.

The reason for this is that while the pressure to sleep builds the longer we stay awake, our circadian clock sends an alerting signal to the brain to counter this pressure. This signal is most powerful about two hours before we normally go to sleep. This accounts for the “second wind” many people claim to experience at night. It also explains why it is easier to stay up late than it is to go to sleep earlier than usual.

This circadian modulation of sleepiness and alertness helped ensure our survival: We are most alert close to bedtime because this is when our former predators, like lions and tigers, are on the hunt. We balance this peak of alertness with a lull when the day is at its hottest and predators are having a lull of their own. The clock in our heads is a truly amazing legacy of our evolution.

Sleep Is an Active Process

My patients often say they have trouble sleeping because they “can’t turn off their brains.” But as long as we are alive and healthy, our brains are not meant to be turned off!

For centuries, sleep was considered a passive, almost deathlike state. But with the advent of modern sleep science, we now know that sleep is an active process. The brain is not turned off but instead goes through predictable cycles of various sleep patterns known as sleep stages. The repetitive combination of sleep stages is referred to as a person’s sleep architecture.

The modern era of sleep science was born in the 1950s when scientists began combining measurements of brain waves with other electrical signals from the body. To help make sense of the measurements, scientists divided sleep into two distinct modes: rapid eye movement (REM) sleep and non–rapid eye movement (NREM) sleep. Since 75 to 80 percent of our sleep time is in NREM, scientists further divide it into three stages that correspond to light, intermediate, and deep sleep (N1, N2, and N3, respectively).

Many significant things occur during NREM, including the physical growth of our bodies and memory consolidation. NREM sleep is even thought to have a key function in the restoration of our brains by resetting our brain synapses. In NREM we have our deepest sleep.

In the N1 period of NREM sleep, we transition from the awake state into sleep. We become much less aware of the outside world as we turn inward. Our eyes close and start to slowly roll back in our heads. In this drowsy state, we may think we are still awake, but anyone watching us can see we are checking out.

In N2, which takes up about half of our total sleep time, we have bursts of unique brain-wave patterns called sleep spindles and K-complexes. Sleep spindles are thought to play a role in the brain forming declarative, or explicit, memories—memories of facts and occurrences that can be consciously recalled, such as specific events in our lives or new information we learned that day.

N3 (commonly referred to as slow-wave sleep) is our deepest sleep. Dominating the first third of the night, N3 takes up 10 percent or less of total adult sleep time (children and teenagers will have as much as 20 percent and sometimes even more N3 sleep). In N3, our breathing and heart rate reach their lowest point. This is the period when it is the most difficult to wake us up. N3 is when our brains secrete the most growth hormone (and likely the time when children’s bones actually grow longer—inadequate sleep is associated with growth problems in children). It, too, is thought to play a role in consolidating our memories and may be a future target for therapies for conditions such as dementia. In N3, odd phenomena such as sleepwalking can occur (see Sleepwalking for more).

REM is the sleep stage most closely associated with dreaming and dominates the last third of the night. Looking solely at brain waves, it is difficult to distinguish REM sleep from being awake, with our eyes open. This implies that the mental activity of looking around while awake and dreaming are similar. (In contrast, when we are awake with our eyes closed, our brain waves look completely different.) Although most of our other muscles stop moving, our eye movements dramatically change as we transition from NREM into REM, with wild, seemingly chaotic movements (hence the name). Our heart rates also fluctuate greatly. If you lead a sedentary lifestyle, REM may be when you reach your peak heart rate on a given day. REM episodes tend to become longer in later sleep cycles, and at the end of the REM bouts, we usually change our sleeping positions and briefly wake up (more on this on No One Sleeps Through the Night).

Each complete sleep cycle—which can include all of the NREM and REM stages—lasts about ninety minutes, although individual cycles vary throughout the night. As shown in the illustration opposite, we sleep most deeply in the early cycles, and REM is longer and more intense in the later cycles. The first sleep cycle is dominated by our deep, slow-wave sleep and contains very little REM sleep. As the night goes on, this mixture changes, so that by the early-morning hours we have very little of that deep sleep, and REM sleep prevails. (This is why we have our longest and most intense dreams just before we wake up to start our day.)

This overall pattern of sleep stages and cycles—your sleep architecture—reflects the quality of your sleep. Many potential clues into people’s sleep problems can be found by analyzing their sleep architecture (for example, see You Should Never Wake Up Tired).

All Sleep Stages Are Created Equal

You might assume that because slow-wave (N3) sleep helps our bodies and minds recover and reenergize, it is the most important, most refreshing sleep of the night. It is important, and it is refreshing, but as discussed in Sleep Is an Active Process, slow-wave sleep makes up only about 10 percent of total sleep time in healthy adults. Does that mean that 90 percent of sleep isn’t as necessary? Clearly this is not the case. If slow-wave sleep was the only sleep you got at night, you would feel the full effects of sleep deprivation.

I’m often asked what the most important sleep stage is. Behind this question, I suspect, is a hope that perhaps there is a stage of sleep we can do without. If we could replace our light sleep with deeper sleep, maybe we’d feel more refreshed when we wake up. It remains to be seen if this is possible to do safely or in any sustained way.

In fact, our brains have some self-correction with regard to our sleep stages. If, for example, we wake up too early and cut off some of our REM time, when we do get a chance to sleep again, we will have more REM sleep than usual. This is called REM pressure and explains why when we are very sleep deprived we may start to dream with our eyes open.

No One Sleeps through the Night

Each of us wakes up about every ninety minutes in our sleep cycle. We toss, stir, change position, sometimes even open our eyes and scan the room. It’s a phenomenon so brief (lasting less than a minute) that in most instances we don’t remember doing it. Not only do we wake up at ninety-minute intervals, but about ten times an hour we have short arousals lasting approximately three seconds each. In all of these instances, your brain is simply doing what it is supposed to do. If we slept straight through for seven or eight hours in a row, the lions and tigers would have picked us off into extinction long ago! Waking up during the night is normal; having trouble going back to sleep is not. No matter how sleepless you feel, make this your mantra: Sleep will come. It’s a natural function of human biology.

Sleep Fragmentation: Stuck in First Gear

More than any specific sleep stage, what matters for refreshing sleep is the healthy continuity of the sleep cycles. When these cycles are disrupted, we use the term sleep fragmentation. Think of the natural mixture of sleep stages as your car’s transmission. The normal progression of shifting gears (moving from one sleep stage to the next) allows you to efficiently obtain a smooth ride (a good night’s sleep). But if your transmission is slipping (continuously starting and stopping your sleep), or you’re stuck in first gear (N1 sleep), you will have an inefficient and choppy ride—you’re not getting the good sleep you need.

Your sleep fragmentation may be out of your control; it could be the fault of a snoring sleep partner or a crying infant, or a painful condition that wakes you in the night. There might not be much you can do to prevent these sorts of interruptions (short of getting your partner to a sleep doctor—see Finding the Right Doctor!). But there are some behaviors you can curtail, such as drinking alcohol or caffeine close to bedtime or taking long naps during the day, to prevent sleep fragmentation.

You Might Not Need Eight Hours of Sleep

An average of seven to eight hours of sleep allows most people to wake up feeling their best, but the optimal number of hours one should sleep depends on the individual and the assumptions that your sleep is normal in quality and its timing is predictable. Ultimately, there’s no magic number of hours you need to sleep each night. Put simply, you need as many hours of sleep as it takes for you to wake up feeling refreshed and remain alert throughout the day. “Short sleepers” seem to have a genetic predisposition to sleep fewer hours than average, while “long sleepers” tend to need more sleep. But if your need for sleep noticeably changes, consult a doctor.

Can I Get By with Less Sleep?

You can’t. Next question.

Seriously, Can I Get By with Less Sleep?

A fortune is waiting for anyone who can come up with a way to give us the restorative effects of eight hours of sleep in only four hours. You may be able to “get by” with less sleep than your body requires, but you are doing only that, getting by. The ability to put off sleep is built into our brains, and we can do it up to a point. We can also skip meals—our physiology has evolved to allow us to temporarily go without the basic necessities. But we do not function at our best in these situations. For optimum mental and physical health, don’t treat sleep like an inconvenience; make it a priority in your life.

What Happens When We Don’t Sleep?

Sleep consumes one-third of our lives, and poor sleep can severely impair the other two-thirds. It’s essential for our health and wellness. Just consider what happens when someone doesn’t sleep: After just twenty-four hours without sleep, a person’s reaction time is similar to that of a drunk driver. When we are fighting to stay awake, we may have brief bouts of sleep called microsleeps, where we might not even realize we have fallen asleep—a potentially dangerous situation if we’re driving or operating machinery. Going even longer without any sleep, known as complete sleep deprivation, makes a person increasingly irritable, emotional, and inattentive. Reaction time is further diminished. After not sleeping for three days or more, a person may start to hallucinate and eventually become delirious.

Suffering from poor-quality sleep for an extended period of time can have the same general effects as complete sleep deprivation, putting yourself and others at risk during unconscious microsleeps. It’s perhaps even more dangerous, because people with chronic sleep deprivation may be unaware of the effects that lack of sleep is having on them. There are many serious physical, psychological, and emotional tolls caused by extended periods of poor sleep. Long-standing sleep deprivation is a stressor on the body, and it is not only associated with a greater risk of obesity, diabetes, hypertension, and cardiovascular disease, but also may increase the risk of cancer and serious neurological conditions such as Alzheimer’s. Significant cognitive deficits can occur, including decreased thinking speed and verbal memory, and even a greater likelihood of forming false memories! Your mood may suffer, you may overreact to stressful situations, and you are more likely to engage in risky and impulsive behavior. If you suffer from mental illness, lack of sleep can exacerbate these conditions. Simply put, we have to sleep well to be healthy.

Sleep and Immunity

If you deprive yourself of sleep, your body’s immune system cannot function at its best. Lack of sleep is a stressor to the body, and the body’s reaction to this stress is similar to its response to low-grade inflammation.

Common wisdom has it that when you feel sick, you should take to your bed. In fact, the release of special immune-system proteins called cytokines—part of the body’s reaction to infection—makes you sleepy, so bed rest is often just what the doctor ordered. If you don’t get the necessary sleep when you’re fighting off an illness, you are adding to the demands of your body to mount an anti-inflammatory reaction. So if you get sick, don’t try to work your way through it.

You should also be prioritizing sleep before