Why We Sleep

The question arises of what exactly is the purpose of sleep.  The short answer is that we don’t know exactly.  However, as the science of sleep progresses, our understanding as to the true purpose of sleep will likely be elucidated.

The fact that in deep NREM sleep physical and mental activity slow down considerably, has led to the reasonable speculation that this phase of sleep serves as a recovery and recharging function. While lower species don’t have REM sleep, almost all species do have NREM suggesting that this phase of sleep has a restorative function. When you’re in NREM you’re recharging your batteries. More specifically, N3 sleep or deep sleep is felt to be the most restorative.

This recharging function is essential because a prime consideration of both the brain and body is energy production and conservation. The brain’s prime goal is survival and its primary defense system, the fight/flight system, uses a lot of energy. Thus the production and conservation of energy is a critical function and it is assumed that NREM fills this role. When people are deprived of NREM sleep they do continue to feel very tired and the body tries to compensate for this on subsequent sleep occasions.

If NREM is about energy conservation, what is REM? Paradoxically REM uses a lot of energy. Perhaps the NREM phase of recovery, mentioned above, is among other things, designed to ensure that there’s enough energy for the REM phase?

Some researchers have argued that because of the high energy output in REM, the energy conservation of sleep overall is pretty small, particularly compared to being awake and just resting. Others have pointed out that if restoration of energy was the purpose of sleep, large animals should require more sleep, but in fact they sleep less.

One view of the energy consuming REM phase of sleep is that its main function is the consolidation of memories formed during the day. This would be crucial for learning and indeed young children do seem to require more REM than adults. One metaphor that might be useful here is that during the REM phase of sleep, the brain is filing away the day’s events. As you open the relevant filing cabinet you will see other associated ideas and other relevant recent events and their associations. If that were the case, our dreams would involve recent events and associations these events have with past experiences. Of course, a lot of dreams are like that. Moreover, the dreamer’s emotional state at the time of the dream would likely shape the dream sequence. So it’s likely that some consolidation of memories occurs during REM sleep and this process helps to explain dreaming.

While the consolidation of memories might be an efficient use of sleep time, it doesn’t appear too critical to functioning. As mentioned above, many species don’t even have a REM sleep phase, and when REM is suppressed it doesn’t appear to affect basic functioning.  However, in humans, when REM is suppressed for while, as in heavy alcohol consumption, there is a period after the REM suppression in withdrawal, during which there is increased amounts of REM sleep, what is called REM rebound.

The evidence is not overwhelming but it is reasonable to assume that NREM sleep serves some energy restoration function and that NREM sleep serves some neural consolidation of memories and learning.

Now we have a better understanding of sleep, it has allowed us to research sleep patterns and determine the type, rate and costs of poor quality sleep, reduced sleep time, or a combination of both. And we have been unable to expose the myth, common until about 20 years ago, that sleep deprivation or poor quality sleep doesn’t cause serious health and economic consequences.

History of Sleep

Galen

History of Sleep
While it seems obvious that humans have slept throughout their history, the way they did so in the past might be different from how we sleep today. For example, in his famous Oddysey written around 800 BC, Homer writes about people having two very distinct periods of sleep at different times of the 24 hour cycle. About three hundred years later, the Greek Alcmaeon observed that blood seemed to be draining from the vessels as we sleep, and assumed that this was a cause of it. Similarly, around 400B.C., Hippocrates noted that bodies cool during sleep and that sleep was therefore characterized by blood retreating from the periphery. Not long after that, Aristotle suggested that sleep was brought on by the warming of the body through digestion. Of course, such an observation would be heavily influenced by what one was eating and drinking and certainly rich foods and plentiful amounts of wine could have people nodding off in no time.

Up until about 162 A.D., the view was that the heart, not the brain, was the center of influence from which all bodily actions were controlled. It was Galen who suggested that the brain was the body’s control center and not the heart, leading to sleep theories that were centered on brain rather than heart activity. Subsequent research suggests that the ancients weren’t totally wrong about the heart. It turns out that there are about 50,000 neurons (brain cells) in the heart that connect with the brain. And there are 500 million neurons in the gut, so perhaps Aristotle’s connection between sleeping and eating wasn’t entirely off the mark.

Despite Descartes’ effort during the Renaissance to divide mind and body, much of the subsequent work focused on the brain’s control of the sleep process. In 1650, Dr. Willis identified the functions of different parts of the brain; in 1929, Hans Berger invented the EEG, which opened the study of the brain and its activity. Later, various areas of the brain were identified with the specific functions of sleep.

Specific identification and treatment for sleep disorders emerged in the twentieth century. In 1903 the first sleeping pill was introduced and in 1930 the first stimulant treatment for narcolepsy was developed. Restless Leg Syndrome (RLS) was first described by a Swedish doctor in 1945 and other conditions were subsequently diagnosed; for example, Dr. Schenk and colleagues first reported on a group of patients who didn’t demonstrate the usual muscle paralysis when in REM. In the last seventy years, there has been even more research, which has been able to identify the different stages and phases of sleep.

Nathaniel Kleitman was one of the pioneers of sleep research, with an interesting personal history. Arriving in New York as a penniless 20-year-old in 1915, he went on to get a PhD and conduct some of the earliest research on sleep and the brain. Using a crude EEG machine that used about half a mile of paper each time it ran a sleep test, Kleitman noticed the different stages of sleep, especially dreaming sleep. In a 1953 paper, he and his colleague, Eugene Aserinsky, called this stage of sleep “Rapid Eye Movement.” Kleitman, who lived to be 104, was fascinated by the concept of wake/sleep cycles and even conducted research on the effects of sleeping in caves and submarines.

One of Kleitman’s proteges, William Dement, continued the tradition of researching the brain and sleep using EEG technology. Dement was interested in sleep dysfunction and has contributed enormously to the diagnosis and treatment of sleep disorders. He is widely considered the father of sleep medicine. He launched what is now known as the American Academy of Sleep medicine and served as president for twelve years. Dement also played bass as a jazz musician and has even played with musical greats, Ray Charles and Quincy Jones.

Dement formed the Stanford University Sleep Disorders Clinic which has been the home to a great deal of important sleep research and some other very influential researchers. One of these is Christian Guillemenault, a prolific researcher, who has written more than 600 academic papers. His most noted work has been on the subject of sleep apnea. He co-opted cardiologists to measure and observe blood pressure and cardiac function in people who have what we now know as sleep apnea. In fact, Guillemenault was one of the first to use the term ‘obstructive sleep apnea’ and was the first to use tracheostomies to free the airways, both providing relief to sufferers as well as convincingly demonstrating the relationship between apnea and cardiovascular function.

Despite the work of these pioneers, sleep medicine research and literature lagged behind that of the other key lifestyle behaviors, like nutrition and exercise. It is only in the past decade has there been a recognition of the importance of sleep, elevating it to a science more in keeping with its importance.

What is Sleep?

 

What is sleep?

Regardless of the various theories, we know that sleeping is a natural function common to all humans and animals, too. We are designed to function on a wake/sleep cycle, although with the advent of artificial light (thank you, Edison for the 1879 introduction of the light bulb), energy boosting drinks, foods and medications, that cycle can be severely disrupted. However, it is clear that daily sleep is the preferred default setting for humans and that sleeping serves some very important functions that underpin health and wellbeing. However, there isn’t a complete agreement on what the functions of sleep actually are.

Sleep can simply be described as a state of reduced sensory and environmental responsiveness and physical inactivity. By comparison, wakefulness is characterized by, sensory perception, thinking, environmental responsiveness and physical activity.

There are various stages of sleep in mammals; the two most prominent are Rapid Eye Movement (REM) and non-REM (NREM) sleep. These two phases are quite different.

In REM sleep, muscles are effectively paralyzed, what is called atonia, and dreaming occurs. Obviously, it’s a good design feature to be paralyzed while dreaming otherwise many of us would be sleep walking and acting out or dreams with physical actions. This atonia is achieved through muscle inhibition by parts of the brain that regulate movement.    Interestingly, there is a sleep disorder called REM Behavior Disorder which is characterized by individuals acting out their dreams.

In REM there is also an increase in breathing and heart rate variability. In addition, in REM the brain also uses a lot of energy, which is important because one theory of sleep is that it is about energy conservation, which might seem paradoxical. There is obviously a lot of mental activity in REM unlike in the other sleep phase, which has earned the REM phase of sleep “paradoxical sleep.”  Core temperature is less well regulated in REM but sexual arousal is common and independent of dream content. In other words, physiology comes first and arousal is experienced, which may or may not be incorporated into a dream’s content.

NREM sleep is characterized as featuring general immobility, regular respiration and heart rate, and slow mental activity. It is divided into three parts:

N1: falling asleep, just in that border between nodding off but still easily awoken

N2: breathing and heart rate slow as you drift off to sleep

N3:  the slow wave delta phase that characterizes NREM. The hallmark feature of N3 sleep is slow, high amplitude delta waves on EEG.  The first episode lasts 45-90 minutes but gets progressively shorter though the night. Children tend to have more N3 sleep than older individuals.

These two phases of sleep are so different that they have led to speculation that there is more than one function of sleep and those functions are represented by these quite different states.

In addition to these separate and distinct sleep states, there is also a typical pattern of sleep in humans as we move in and out of these different phases.

Brain Wave Activity

Delta: 1-4 cycles per second: Deep sleep. Typical NREM phase

Theta: 5-8 cycles per second: Conscious, but low level of brain activation, e.g. meditation.

Alpha: 9-13 cycles per second: Relaxed wakefulness

Beta: 14-30 cycles per second: Active processing, stress.

Sleep Cycles

Sleep occurs in cycles that typically last 90 minutes; the ultradian sleep cycle.  Sleep proceeds from NREM phase to a REM phase, about five times a night. There is typically more NREM in the earlier part of sleep and more REM in the later part of the night or early morning. This is why people commonly awaken during their dreams.  REM accounts for between 20% and 25% of total sleep time.

The way in which a person cycles through these phases, as well as the quality of the stages of sleep, determines the healthiness or otherwise of sleep. There are almost 80 distinct types of sleep disorders, which reflect different dysfunctions in the stages and phases of sleep.