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Sleep is an activity that, until recently, didn’t get much respect.

For centuries we assumed that nothing happened when we were asleep because the brain and body weren’t doing anything but resting. It’s only been in the last 80 years or so—since the development of technologies like EEG and MRI—that we have been able to study sleep real time. It’s been an eye-opener to learn how incredibly active our brains and metabolisms are while we aren’t. But even though we are developing a greater appreciation for how complex and important it is, what actually happens during sleep is mostly a mystery.


In theory at least, we spend about 2,500 hours a year sleeping, but we don’t know a lot about what happens while we are doing it. Fortunately, that doesn’t matter very much because most of what goes on is automatic and we don’t have to do anything to make it happen, except create an environment where sleep is possible (no small task but we’ll get to that later).

Sleep has a mixed reputation. Shakespeare was a big fan. He referred to it as “balm of hurt minds” and “the chief nourisher in life’s feast.” Thomas Edison, not so much. The inventor famously avoided sleep (see pull quote). Fortunately, as researchers accumulate more information about sleep, the pendulum is moving away from the militant anti-sleep attitudes of Edison toward a more appreciative understanding of its value.

Sleep does not serve a single purpose. It’s a human body equivalent of duct tape because it does so many things. It’s necessary for the consolidation of learning and memory, proper immune functioning, general physiological housekeeping and maintenance, and emotional functioning to name just a few.

All kinds of metabolic activities happen when we sleep. We create a certain amount of jumble and disorder in our brains during a long and busy day and one of the activities that seems to occur when we sleep is that the brain has a chance to clean up the mess. For example, recent research suggests that during sleep the space between cells in the brain may expand slightly to allow greater flow of cerebral spinal fluid to flush out the debris.

Sleep is important in learning and memory formation. We acquire information while we are awake, so people who are sleep-deprived generally have less ability to concentrate and learn.

But the actual process of sleeping also appears to be important for the consolidation of learning.

Studies show that after exposure to a complex problem, those who were allowed to work on the problem and then get eight hours of sleep were 60 percent more likely to find a novel solution than those who did not get the eight hours of sleep.

Sleep also facilitates the proper functioning of our immune systems. In a recent study, researchers evaluated the response of students to a hepatitis C vaccination. A group given the vaccine and then allowed a normal night’s sleep was compared with a group given the vaccine but deprived of sleep for 24 hours. Antibody counts four weeks later showed the normal sleep group had a much stronger response to the vaccine, with antibody levels that were 97 percent higher than the sleep deprived group.

And on it goes. Study upon study, in both animals and humans, support the theory that sleep is essential to give the body a chance to repair and rejuvenate itself. In fact, it appears that many important processes like muscle growth, tissue repair, protein synthesis, and hormone release occur mostly, and sometimes only, when we sleep.


Our need for sleep is mostly hard-wired, so we don’t control it nearly as much as we think we do. Although our bodies might be telling us that we’re hungry, it is possible to override the hunger pangs and make a choice not to eat, and we can do that until we starve to death. We can try not to sleep, too, but eventually the body takes over and we will sleep. Ee don’t actually know how long a human can live without sleep but we do know that animals deprived of all sleep lose their immune functioning and die within weeks. Sleep researchers generally agree that adult humans require between seven to eight hours per day to function optimally although about five percent require a bit less or a bit more. There are two mechanisms that regulate sleep—our homeostatic sleep-wake cycle and our circadian system.


Circadian rhythms are biological processes that follow a daily schedule. Circadian clocks control their timing. Our circadian rhythms regulate multiple body functions like hunger, alertness, cardiac functioning, and our immune system.

Metabolic processes within the body produce circadian rhythms, but signals from the environment also a_ect them. The main external cue influencing circadian rhythms is daylight.

Although we probably have biological clocks in different organs throughout the body, there appears to be one main circadian timepiece. This clock — also known as the suprachiasmatic nucleus or SNC — is located in the hypothalamus. Its function is controlled by light from the environment and by genetics. There are photoreceptors in our eyes that only respond to changes in light and dark and their primary job is to regulate our circadian rhythms. Sunlight resets the clock every day to keep it to a roughly 24-hour cycle.

There are, however, variations in circadian cycles that dictate, for example, whether we are “morning people” or “night owls” and these are determined by genetics.

For most of history, our biological clocks were regulated by sunlight although today they are strongly influenced by artificial light, too. The majority of us in the same “time zone” will have our clocks set to about the same time—that is, we are mostly programmed to get sleepy at the same time—when it gets dark. And, we are mostly programmed to wake up when it becomes light.

Circadian rhythms regulate metabolic functions by releasing specific hormones in varying amounts on a 24-hour cycle. Our sleep cycle is influenced by one of these hormones—melatonin—which causes us to feel sleepy. When it gets dark and there is less light input to the SNC, the release of melatonin goes up. When dawn breaks and there’s more light input, melatonin production declines.

It’s common to experience problems when we disrupt our circadian cycles—for example, when we experience jet lag or work the night shift. But causes of circadian disruption can also be more subtle.

Researchers have found that even the slight variation caused by the springtime switch to daylight savings time—when we lose an hour of sleep—results in more workplace and car accidents.


The human body maintains its equilibrium by constantly changing. We have highly sophisticated monitoring systems to keep us in balance—when we are dehydrated we become thirsty; when we’ve been awake long enough, we get sleepy—that’s homeostasis.

The strength of our homeostatic sleep drive is determined by how much sleep we have had in the past 24 hours and how much sleep debt we’ve accumulated. There are specific areas in the brain responsible for either maintaining alertness or for promoting sleep. When the alertness centers are active, they inhibit the sleep-promoting areas and vice versa. Similar to a battery that holds a set amount of charge, the human body is designed to need about eight hours of recharging sleep after 16 hours of being awake. When we’ve had sufficient sleep, the balance shifts from the sleep-promoting center to the alertness center and we wake up. However, as the day wears on our alertness dims. We experience increasing tiredness, the low battery light on our internal dash begins to flicker, and after about 16 hours or so, we are ready to sleep again. How do we know the battery is running low?

There are sleep-regulating substances that occur naturally in the body and fuel our homeostatic sleep drive. Adenosine, a neurotransmitter, is a by-product of energy consumption in the brain. One of its functions is to inhibit the neurotransmitters that promote wakefulness. Adenosine levels build up in the brain the longer we are awake and studies have shown that high levels of adenosine cause sleepiness. One reason that coffee is the second most traded commodity in the world (after oil) is that caffeine blocks the action of adenosine and helps maintain alertness.


Prior to 1929, it was commonly held that sleep is a passive process, but the invention of the electroencephalogram (EEG)—which records brain waves—proved that the brain is very active during sleep. The identification of rapid eye movement sleep—REM sleep—in the 1950s further illuminated the complexities of sleep. Today, we know that “a good night’s sleep” is not a single process, but a sequence of different types of sleep that have different characteristic and functions.

Sleep is divided into two main cycles, REM and non- REM or NREM sleep. NREM sleep is further divided into three cycles: N1, N2, and N3. REM sleep is characterized by rapid brain waves and lots of eye movements. It is also referred to as paradoxical sleep because although our brains are very active—similar to how they are when we are awake—our bodies are paralyzed and we can’t move much. REM’s other claim to fame is that it is the sleep cycle during which we do most of our dreaming.

NREM sleep is characterized by slower, synchronized brain waves which gradually become more rhythmic as we descend from the lighter sleep N1 cycle into the very deep sleep of the N3 cycle, during which we are pretty much dead to the world and hard to wake up. We seem to have dreams during NREM sleep, too, but these are different from and not so frequent as those during REM sleep.

We alternate between REM and NREM sleep in roughly 90-minute cycles, four or five times throughout the night. We seem to have more NREM sleep earlier in the night and longer REM cycles closer to night’s end. The pattern of our sleep cycles can vary day to day and is influenced by multiple factors, for example our age, how tired we are, how much sleep debt we have accumulated, and what we ingested or did before bedtime, to name just a few.

What happens during the dfferent sleep cycles isn’t well understood, but it seems that the dierent sleep cycles serve different functions and may vary in length depending on the specific needs of our bodies. For example, people who have been recently sleep-deprived may have longer deep-sleep cycles. Other evidence suggests that emotionally charged memories might be processed during different cycles than more neutral information.


Our sleep debt is the dfference between how much sleep we need and how much we get. Pulling an allnighter or traveling to Europe causes a temporary problem that can be fixed by a few nights of good sleep. What is more concerning, according to sleep experts, is the overall reduction in the hours we sleep. Although there is some debate about this, it seems that all of us—adults and children—are getting about 60 to 90 minutes less sleep per day than we did 100 years ago. It is this chronic shortage—one that can’t be made up by a two-week vacation or sleeping in on Saturday— that’s thought to contribute to an increased risk of all sorts of chronic disease such as obesity, diabetes, heart disease and hypertension, possibly ADHD, mood disorders, and on and on. Sleep expert Matthew Walker, director of the Center for Human Sleep Science at UC Berkeley, bluntly states,“The number of people who can survive on five hours of sleep or less without any impairment, expressed as a percent of the population and rounded to a whole number, is zero. No aspect of our biology is left unscathed by sleep deprivation. It sinks down into every possible nook and cranny.”


A sleep debt can obviously result from getting too little sleep and there are lots of reasons why we are sleeping less. One very big one is that we can. If we are on a camping trip in the wilderness, away from electronics and electricity, we generally get more sleep—it’s not safe to stumble around in the dark and there’s not much else to in the absence of light anyway.

When we return to civilization, however, there are abundant distractions, and the motivation to sleep diminishes.

Environmental factors can disturb our sleep. A major one is our increasing exposure to the shorter wave or “blue” light from our computers, cell phones, electronic readers and televisions. Our circadian system interprets this blue light as daylight and as a result, postpones releasing the melatonin required to prepare the body for sleep. A recent study done by Harvard sleep expert and researcher Charles Czeisler compared subjects who read from regular print books before going to bed with those reading from a light-emitting e-book. The e-book readers released less melatonin, were less sleepy, took longer to fall asleep and were less alert the next morning.

As a society we cram more and more into our days, so we have correspondingly less time for sleep. According to Stanford sleep expert William Dement, since 1969 working mothers with children have added 241 hours to their annual work and commuting schedules. This averages out to an 11 percent increase in the length of the workweek, or about five hours a week. The rest of us have only added 158 hours, but that still adds up to a full month of work. Often, we pay for that increase by decreasing the time we spend sleeping.

There are cultural reasons that prevent people from sleeping. A major one is that we don’t value sleep and we don’t acknowledge or perhaps don’t even recognize the problems created by the lack of it.

Dr. Charles Czeisler commented on this trend in an interview in the Harvard Business Review. “We now know that 24 hours without sleep or a week of sleeping four or five hours a night induces an
impairment equivalent to a blood alcohol level of 0.1 percent . . . yet we continue to celebrate people who sacrifice sleep. The analogy to drunkenness is real because, like a drunk, a person who is sleepdeprived has no idea how functionally impaired he or she truly is.”

We can also build up a sleep debt if we routinely get poor quality sleep. Many things can interfere with how soundly we sleep. For example, we often sleep less well as we age, when we sleep in an unfamiliar bed, or when we drink alcohol or caffeinated beverages too close to bedtime. Medical problems like multiple sclerosis can prevent sound sleep because people experience symptoms—for example, pain or leg spasms—that frequently wake them up. This prevents them from entering deep sleep, so even if they are sleeping long enough, they may not be sleeping well enough.

And sometimes, in the process of trying to prevent a sleep problem, we might actually create one. Ideally, we want our sleep drive to be synchronized with our circadian rhythms because we sleep better when these processes are aligned rather than in competition. Irregular sleep patterns, or chronically playing catch-up, interfere with this synchronization. It’s a common  contemporary practice to burn the candle at both ends, shorting ourselves a few hours of sleep each night during the workweek, with the plan of sleeping in on Saturday and Sunday to repay the debt. German sleep researchers use the term “social jet lag” to describe the impact of this weekly disruption of the internal clock, which they believe is equivalent to traveling across three to four time zones twice a week. They have also noted a positive association between the magnitude of social jet lag and body mass index, adding further he to the theory that the disruption of circadian cycles contributes to weight gain.

And, speaking of playing catch-up, what about naps? Our circadian sleep cycle is biphasic, which means we have two times during a 24 hour cycle when we have the strongest drive to sleep. Those times are roughly between 2 and 4 a.m. and 2 and 4 p.m. In some cultures it is common practice to take a mid-day siesta, the timing of which coincides with the lull in our alerting system, so an hour nap can take the edge off the growing drive to sleep and extend the time until we need to go to bed. People who routinely take siesta mid-day usually have a shorter nighttime sleep cycle. For any of us, a brief nap (30 to 60 minutes) can be a good way to catch up when we don’t sleep well at night.


That’s a good question. Sleep is a very complex process. Chronic sleep disturbances, especially in combination with a chronic disease like multiple sclerosis, are usually complicated problems to manage. They seldom have quick and easy solutions. The first step to managing them is to recognize that good sleep is essential to our health. Figuring out how to get some needs to be at the top of our list of problems to solve. Frequently it’s just an afterthought.

How do you know if you have a sleep problem? If you never wake up without the alarm clock, if you are always tired, if you doze o when you sit down, if you often find yourself too tired to think, if you are chronically cranky and irritable—all these are signs that you may not be sleeping well enough.

As described in the following article by Tom Stewart, there are a number of fairly simple interventions to try at home that oen improve sleep problems. There are lots of good books available—for example, the Harvard Medical School Guide to Getting a Good Night’s Sleep. There are online CBTi (cognitive behavioral therapy for insomnia) programs available to help you plan and organize your sleep efforts.

It’s important to talk with your healthcare provider about your sleep concerns and to be persistent if these are not given adequate attention. Medication can be helpful, but it’s often a partial solution at best. Referrals to sleep experts for further evaluation are often essential.

While you are working your way through that list, though, there are a few other things to try. You can turn off your electronic devices 90 minutes before bedtime to give your internal melatonin time to work. There are also millions of gadgets out there that are marketed as solutions for insomnia so, after you get a blue light filter for your computer screen (available for free at justgetflux.com) you can amuse yourself by searching for these. You can replace some lightbulbs with not cheap but still relatively inexpensive “goodnight” light bulbs. And, if you still have diffculty falling asleep, check out the “Sleep With Me Podcast, The Podcast that Puts You To Sleep.” This is a free podcast of “lulling, droning, boring bedtime stories to distract your racing mind,” and they do just that. They are really boring, as advertised. And they are also quite amusing.

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