Why We Sleep
Sleep is as vital to human survival as food and water. We spend approximately a third of our lives in its embrace. Scientists from the time of the ancient Egyptians to today have studied sleep and tried to determine its exact function and interpret what happens inside the brain during slumber. Concrete answers have proved elusive and there are many elements of sleep that continue to baffle scientists.
Here’s what they do know: everybody does it. In fact, all animals sleep and it is necessary for their survival. Laboratory studies found that rats, which have a normal life expectancy of several years, will survive an average of only five months if deprived of REM sleep.
Rats deprived of all sleep will lose the ability to maintain body temperature and die after about three weeks. In humans, sleep deprivation suppresses the immune system.
What’s not clear is why sleep is necessary – here are some theories:
Adequate sleep is physically necessary to maintain proper brain function when it comes to speech, memory, creativity, and cognitive thinking. Some researchers have suggested that brain synapses are pruned and reorganized during sleep.
There are various thoughts on “brain maintenance” during sleep. Memories and events can be consolidated and synaptic strengths may be rebalanced. It is thought this improves the efficiency of the brain. This process can be compared to hard drive defragmentation.
Gene Expression and Restorative Functions
There are certain genes that are only turned on during sleep. Research conducted at the University of Surrey, England has revealed that “insufficient sleep – less than six hours a night –
affects the activity of over 700 of our genes.” These genes are responsible for managing stress and inflammation, among other things.
Some researchers have suggested sleep allows the body’s internal processes to rejuvenate from its daily activity. Other scientists are not strong proponents of this theory, the reason being that the amount of energy expended during sleep to keep our cellular functions running is almost identical to the amount of energy expended while awake. Sleep’s purpose, then, appears more fundamental and complex than simply recharging energy levels.
We know that sleep happens in recurring stages or cycles that last from 90 to 110 minutes. There are two types of sleep: rapid eye movement (REM) sleep and non-REM sleep. These stages are characterized by physiological changes in brain wave activity, breathing, heart rate, and body temperature that fluctuate depending on the stage of sleep.
As the sleep session progresses, however, the time spent in deep Non-REM sleep decreases and the amount in REM sleep increases.
Non-REM sleep has four stages: light sleep, true sleep, and two levels of deep sleep. They are also simply referred to as Stage 1, Stage 2, Stage 3, and Stage 4 sleep.
Light sleep is just when you feel yourself drifting off; it is that half awake and half asleep sensation. The body’s muscle activity slows down and some individuals may experience involuntary muscle twitching. It is easy to wake up during the light sleep stage.
Approximately ten minutes into a light sleep the true sleep stage starts, which last around 20 minutes. During this time our breathing and heart rate begins to slow down. This period accounts for the largest part of human sleep.
The third and fourth stages are deep sleep. During Stage 3, the brain starts to emit delta waves. Delta waves are high amplitude and low frequencies, meaning the waves are large and slow.
The lowest levels of breath and heart rate also accompany Stage 3 sleep. During Stage 4 sleep, breathing becomes more even and rhythmic and there is very little muscle movement.
It is not easy to wake up from deep sleep; individuals woken during this stage often feel as if they’ve been drugged and are disoriented, needing a few moments to know where they are and what is happening around them. Sleepwalking and bed-wetting occur during deep sleep.
REM is the fifth phase of sleep and typically starts between 70 and 90 minutes after we fall asleep. People typically have three to five REM episodes a night, depending on how long they sleep.
During this stage our brains are very active; some experts believe our brains may actually be more active during REM than when we are awake. And although breathing and heart rate increase, the muscles are rendered inactive and researchers theorize it is to prevent our bodies from flailing while dreaming, as most dreams occur during REM sleep. After REM sleep ends, the cycle begins again with stage 1 sleep.
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Characteristics of Sleep
When we sleep we lose conscious awareness of our surroundings so we can’t directly explain what happens. Instead, researchers have relied on observing changes in behaviour and responsiveness to help define sleep. There are some basic characteristics that have become apparent:
- Sleep is a time of reduced activity.
- There is a typical body position associated with sleep—people lay down while horses stand.
- Sleep decreased responsiveness to external stimuli.
- Compared to other states of reduced consciousness like hibernation or coma, the sleep state is mostly easy to reverse.
For hundreds of years, scientists believed that the brain was inactive during sleep but research since the middle of the twentieth century has revealed that the brain remains active during sleep.
While there is a progressive decrease in neural activation as sleep progresses from stage one to stage four, patterns of brain activity during REM sleep are more random and variable, similar to what is seen when we’re awake.
Sleep is not uniquely human and scientists define sleep the same way for all mammals, as well as other animals, although there are differences between species. In humans the entire brain is involved during sleep; no part is exempt.
But dolphins and whales need to maintain consciousness while they sleep so they can go to the surface periodically to breathe. To accommodate their environment, sleep in these marine mammals happens in one brain hemisphere at a time.
Scientists also define sleep in other physiological terms. Humans are warm-blooded so our body temperature remains constant when we’re awake regardless of our environment through responses such as shivering, sweating, and increased blood flow.
But right before falling asleep, our bodies lose some heat to our environment. It’s believed that minor cooling-off helps induce sleep. And while we sleep our body temperature is a degree or two lower than when awake, meaning we need a little less energy to maintain our body temperature.
Breathing becomes very regular during Non-REM sleep then goes back to a more variable pattern during REM sleep. Our cardiovascular activity also exhibits some significant changes, with an overall reduction in heart rate and blood pressure during
Non-REM sleep. Then again, during REM a more pronounced variation in cardiovascular activity is seen. While scientists know these particular variations happen, nobody has discovered exactly why they do.
Technology provides a window into the world of sleep. By using an electroencephalogram (EEG) to measure electrical activity in the brain; an electrooculogram (EOG) to measuring eye movement; and an electromyogram (EMG) to measure muscle activity researchers can compare patterns for wakefulness, REM sleep, and Non-REM sleep.
The EEG recorded during REM sleep shows very fast activity that is more random than what is seen Non-REM sleep. Instead, it is more similar to the EEG taken when the patient was awake.
REM sleep is marked by bursts of rapid eye movements, periods of inactivity, then another burst of movement, and so on. Just as when awake, both eyes move together in the same direction.
A common belief among scientists is that the eye movements of REM sleep correspond to the visual images of dreams. But why they happen and if they have a function remains unknown.
Lastly, the EMG shows that while muscle tone is normal in Non-REM sleep, humans become practically paralyzed in REM sleep as far as skeletal mobility.
Every now and then a muscle will twitch but by and large, the EMG reading remains a flat line during REM. However, other muscles responsible for the heart, diaphragm, and eyes remain functional as do the smooth muscles found in the intestines and blood vessels.
Through the use of electrical stimulation, researchers have identified specific areas of the brain where sleep is generated. The basal forebrain, including the hypothalamus, controls Non-REM sleep and maybe the part of the body determining if we’re getting enough sleep.
The pons in the brainstem is critical for initiating REM sleep and during that phase sends signals to the cerebral cortex, the region of the brain responsible for most of our thought processes, as well as the spinal cord, causing the temporary paralysis that occurs during REM.
Just as certain animals are nocturnal, humans are naturally diurnal; they sleep when it is dark. This inherent behavior is part of our circadian rhythms; circadian rhythms are physical, mental, and behavioral variations or changes that approximately follow a 24-hour cycle based primarily on light and darkness.
Almost all living things have circadian rhythms that are controlled by biochemical reactions oscillating within a period of 24 hours (circadian or biological clock).
Circadian rhythms play an integral part in determining human sleep and waking patterns. The suprachiasmatic nucleus (SCN), which is located in the hypothalamus (above the optic nerve), is known as the body’s master clock.
It is composed of nerve cells that control the body’s various biological clocks and keeps them in sync. The SCN controls the production of the hormone melatonin which is what makes you sleepy. When there is less light, the SCN tells the brain to make more of the hormone melatonin so you get drowsy. Melatonin is made from serotonin so low levels of serotonin can lead to sleep disorders.
Research shows that your body naturally produces melatonin, a hormone that tells your brain it’s time to go to sleep. In other words, the primary function of this hormone is to let the body know when it is time to go to sleep so that it will relax and sleep easily.
The pineal gland produces melatonin, but it is also produced in other places, including the ovaries, bone marrow and gastrointestinal tract. Due to its benefits, some natural supplements such as Resurge contain melatonin as their primary ingredient.
Although more research is needed, current evidence suggests that melatonin can be useful in helping people get to sleep. When taken for short or long periods of time, melatonin supplements appear to be safe for adults, according to studies.
Resurge supplements might allow your body to absorb enough melatonin to maximize your natural melatonin production. Apart from that, it is also marketed as a weight-loss supplement because epidemiological studies show that insufficient sleep is associated with a higher risk of obesity.
However, since the supplement industry is barely regulated, you might want to read some reviews for the Resurge supplement before making any purchase.
Cortisol is another hormone that plays a role in sleep. Cortisol, produced by adrenal glands, is known as the stress hormone. It is secreted in response to stress or exercise. Elevated cortisol levels at night can interfere with sleep.
At night cortisol levels usually drop to accommodate restful sleep, and then increase during waking hours. Excessive stress keeps the levels high, interfering with sleep, which in turn can result in more stress, increasing the levels even more.
This self-reinforcing cycle is something we will see again and again when examining different aspects of sleep.
Neurons in the brainstem, which connects the brain with the spinal cord, produce neurotransmitters including serotonin and norepinephrine that keep certain areas of the brain active while we are awake. Elevated levels of neurotransmitters have a detrimental effect on sleep.
Studies also indicate that the chemical adenosine builds up in the blood while we’re awake and when the levels get high enough it causes sleepiness. Then while we sleep, adenosine gradually breaks down to waking levels and the cycle begins all over again.
All of these processes happen simultaneously so the body and brain are in a constant juggling act to keep everything in balance so that we get the sleep we need.
Sleep Of Various Animals
Sleep behaviors vary across animals. Insects exhibit periods of rest that resemble sleep in more complex organisms. Numerous studies on fruit flies have affirmed their well-defined circadian rhythms. Their neural activity while resting is similar to the slow-wave sleep in vertebrates.
A study at the University of Pennsylvania documented a sleep-like state in roundworms that seemed to facilitate changes in nerve cell synapses, making a case for the theory that sleep is necessary for brain plasticity—the ability for the brain to reorganize based on new experiences.
Conversely, mammals deprived of sleep show disruption of synaptic changes that are needed for the brain to grow and change.
Reptiles also show changes in brain activity that are associated with behavioural changes. This is suggestive of sleep although REM sleep has not been definitively demonstrated. Even amphibians and fish seem to have sleep-like periods of reduced activity.
While circadian rhythms exist in all plants and animals it’s not been determined if sleep exists in very primitive animals like the octopus or if there is a state of restorative function that could be considered sleep in unicellular organisms.
Most animals sleep, although the patterns, habits, postures, and places associated with sleep varies among species. The common denominator, says Irene Tobler, a researcher at the Institute of Pharmacology at the University of Zurich, is that “most animals, and probably most living organisms, exhibit a circadian rest-activity rhythm.”
Mammals’ sleep patterns generally alternate between Non-REM and REM sleep states in a cyclic fashion, as do birds, although their cycles are much shorter. For example, in a bird, the REM state may last nine seconds. Nor do birds lose muscle tone during REM sleep, which otherwise would be problematic since they perch while sleeping.
These observations led some scientists to suggest that REM sleep may be a later evolutionary development related to warm-blooded animals.
Sleep in humans is timed to the earth’s circadian cycle of light and dark. Many mammals including humans are diurnal, meaning we are active during the day and sleep at night. Others, like rats, are nocturnal.
Sleep Cycles and Light
According to a report in the Environmental Health Perspective journal, “When people are exposed to sunlight or very bright artificial light in the morning, their nocturnal melatonin production occurs sooner, and they enter into sleep more easily at night. Melatonin production also shows a seasonal variation relative to the availability of light, with the hormone produced for a longer period in the winter than in the summer.”
Modern-day sleep is notably different from sleep from just a few hundred years ago. Before there was electricity people slept in two distinct phases, sometimes called first and second sleep and it was primarily governed by the sun’s natural light. People would fall asleep shortly after the sun went down, sleep for about four hours and then wake up for between one to three hours.
During that time they would either stay in bed resting or thinking or procreating or get up and do household chores or even go visit neighbors. Then they would go back to sleep for approximately another four hours until it got light out.
This was an established sleeping pattern for literally thousands of years—in The Odyssey, Homer refers to “the first sleep.” But that all changed with the invention of electric light. It is interesting to note, though, that studies have shown that when put in an environment that simulated a world without electric light, volunteers quickly reverted back to a first and second sleep pattern, suggesting it remains an ingrained natural preference.
One of the most notable characteristics of sleep is dreaming, which remains perhaps the greatest mystery involving sleep. Philosophers and scientists have debated dreaming since the dawn of human civilization. There is no consensus, though, as to why we dream.
People spend around two hours a night dreaming. Visually vivid dreaming occurs primarily during REM sleep, although people also have dreams during non-REM sleep. Notably, night terrors actually occur during Non-REM sleep.
REM sleep stimulates areas of the brain used in learning, which would explain why infants spend so much more time in REM sleep than adults and why college students report being able to retain information better when they study right before going to sleep.
In one experiment, researchers found that people who were taught a skill and then deprived of non-REM sleep could recall what they had learned after sleeping, while people deprived of REM sleep could not.
Some experts propose that dreams represent the replay of the day’s events as part of the process of memory formation, while others believe dream content is simply the result of random brain activity—a view that is generally viewed as too simplistic.
A study conducted by researchers from the University of Michigan and Tulane University led the team to conclude “there are clear cognitive and behavioral ramifications [from dreaming] due to the fact that while asleep our mind not only continues working but acts in such a way that we are necessarily thrust into various virtual scenarios.
The processing of dream content, which consists of variations in scenarios encountered during daily life in which we interact with the physical and social world, is bound to influence our cognitive capacities and subsequent appraisal of real-world content.”
In other words, dreams help shape our view of the world and our responses to it. In that regard, getting better sleep would seem a way to both promote REM sleep and to get the greatest benefit from our dream states.
Sleep Deprivation and Sleep Duration
Eight hours is considered a general approximation of a good night’s sleep, but sleep needs will vary. A poll conducted by the New York Times Magazine found that 14 percent of respondents got less than six hours of sleep a night; 25 percent slept between six and seven hours; a third slept eight hours; one out of five slept between eight and nine hours, and six percent slept more than nine hours a night.
Some of this variation is biological—different people have different sleep needs. Some adults need as much as ten, others are as little as five hours a night. By contrast, infants generally need 16 hours or so and teenagers tend to require nine or more hours of sleep to be completely alert.
Some of the sleep duration variations are caused by factors such as culture, occupation, home life, economic status, age, geography, and physical health.
Research by the Organization of Economic Development (OECD) looking into measuring what constitutes leisure time, found that the French on average spends the most time sleeping and Koreans the least.
The average French person sleeps more than an hour a day longer than the average Korean. So while there may be a strong biological component to sleep, in modern society, at least in Developed Country cultures, some time spent sleeping has become a leisure time activity.
Given the variation and complex combination of factors affecting sleep, how can we determine sleep need? Jim Horne from Loughborough University’s Sleep Research Centre puts it simply, “The amount of sleep we require is what we need not to be sleepy in the daytime.”
If we don’t get enough sleep, a “sleep debt” builds up that eventually has to be paid. The amount of sleep a person needs increases as their sleep debt grows. We may not be aware of the sleep debt we’re accruing because we’re still able to function but scientists can measure our subtle but growing cognitive and motor impairment.
While caffeine and other stimulants can mask or help counteract some of the effects of sleep deprivation, they are not a substitute for sleep. There’s a misconception that older people don’t need as much sleep. They do. But what happens is that they often don’t get as much as they should.
Generally, people tend to have less deep sleep as they get older, and nearly 50 percent of all those older than 65 develop sleeping issues such as insomnia. It’s unknown whether diminishing deep sleep is a function of ageing or if it’s a symptom of an underlying medical problem common in elderly people or a reaction to the medication and other treatments provided for typical ailments or conditions.
There has been researching done as to whether melatonin levels are lower in elderly people, and if so, what the cause would be. I do not find a consensus in this regard.
Experts offer this straightforward rule of thumb: if you feel drowsy during the day you haven’t had enough sleep. If you routinely fall asleep within minutes of lying down, you very likely are suffering from severe sleep deprivation.
Overly tired individuals may experience microsleep, which is a very short period of sleep that occurs when people are awake. It can last from less than a second to several seconds.
Researchers at the University of Wisconsin-Madison found that some nerve cells in a sleep-deprived yet awake brain can briefly go off-line into a sleep-like state while the rest of the brain appears awake.
Dr. Chiara Cirelli, professor of psychiatry, says, “Even before you feel fatigued, there are signs in the brain that you should stop certain activities that may require alertness. Specific groups of neurons may be falling asleep, with negative consequences on performance.”
Prior to the discovery of micro-sleep, scientists believed that sleep deprivation affected the entire brain, but Cirelli says, “We know that when we are sleepy, we make mistakes, our attention wanders and our vigilance goes down. We have seen with EEGs that even while we are awake, we can experience short periods of micro sleep.”
People are not aware that they are experiencing micro-sleep, which makes sleep deprivation that much more dangerous. Driving simulators show that the hand-eye coordination of sleep-deprived people is as bad—or worse—than someone who is legally intoxicated. Moreover, sleep deprivation exacerbates alcohol’s effects so a drink or two will cause much greater impairment in a tired person than someone who is well-rested.
The U.S. National Highway Traffic Safety Administration estimates that driver fatigue is responsible for an estimated 100,000 accidents and 1500 deaths each year. The U.S. National Sleep Foundation strongly warns that if you have trouble keeping your eyes focused, can’t stop yawning, or can’t remember driving the last few miles then you are probably too sleep-deprived to drive safely.
Pullover and call someone to come get you or take a cab. More than 40 million Americans are afflicted with chronic sleep disorders and 20 million more experience periodic problems, all of which can lead to sleep deprivation.
The International Classification of Sleep Disorders lists more than 80 sleep disorders including insomnia, sleep apnea, restless legs syndrome, and narcolepsy. Research also suggests that sleep loss may cause weight gain because the chemicals and hormones that play key roles in controlling appetite and weight gain are released during sleep.
Sleep apnea occurs when there are one or more pauses in breathing while sleeping. It’s marked by heavy snoring and blocked airways. When the blood oxygen level falls enough, the brain wakes the person up enough to tighten the upper airway muscles and open the windpipe. This can be repeated hundreds of times a night.
The constant awakenings leave people chronically sleepy. Sleep apnea is associated with high blood pressure, irregular heartbeats, and an increased risk of heart attack and stroke. People with chronic apnea are up to three times more likely to have automobile accidents and in severe cases the condition could lead to sudden death from respiratory arrest during sleep.
Restless legs syndrome (RLS)
RLS is a common deep sleep disorder that causes tingling or pins-and-needles sensations in the legs, prompting constant leg movement to alleviate the discomfort. These movements occur every 20 to 40 seconds and cause repeated awakenings. As many as 12 million Americans suffer from affliction, which leads to insomnia at night.
Narcolepsy is a condition where a person suffers bouts of sleep, regardless of the amount of sleep they get at night. The spells can pass after a few moments or last up to a half-hour. Narcolepsy tends to begin during adolescence and is usually hereditary.
How Sleep Can Affect Testosterone Levels
Sleep deprivation affects more than motor skills; it can also cause hormonal imbalances. For example, testosterone (T) is male hormone that provides vigor, muscle power, and a sex drive.
Too little sleep can lower T levels, leading to an assortment of issues. The production of T increases during sleep so the more restful sleep men get, the higher their testosterone levels.
Research suggests the poor sleeping habits can dramatically lower testosterone levels in healthy men in as little as one week. A study conducted at the University of Chicago Medical Center found that the testosterone levels of men who slept less than five hours a night for one week was between 10 to 15 percent lower than when they were fully rested, which led to lower energy, poor concentration, fatigue, decreased strength, and an overall loss of wellness.
Sleep deprivation has many negative effects on the body, including:
- Lack of mental acuity
- Compromised immunity
- Feelings of tiredness
- Increased risk of falling asleep while driving
- Weight gain
- Hormonal imbalances
- Rapid heart rate and other potentially dangerous health issues
- Increase in stress
- Anger or depression
I will remind the reader that individual sleep needs will vary and reactions to changes to sleep habits will vary based on the individual.