What Is Sleep?

Sleep is a big part of everyone’s life since we spend approximately a third of our lives sleeping. Getting sufficient quality sleep is essential for our survival, no less so than food and water. Without sleep we are unable to build or maintain the neuronal pathways and connections in our brains that enable learning and the formation of new memories. Without sufficient sleep we have difficulty focusing and our reactions are slow.

Sleep is vital if we want our brains to function well. We need sleep so that our brain neurons (nerve cells) can communicate. Even when we’re sleeping our brains and bodies remain remarkably active. Recent studies indicate that sleep acts as somewhat of a housekeeper, removing toxins that build up in our brains during the waking hours. 

There is no question that we all need sleep, but  its exact biological purpose is a mystery. Sleep impacts nearly every system and type of tissue in our bodies – everything from the heart through the lungs, brains, immune systems, resistance to diseases metabolisms, and mood to our disposition. Research studies show that when people chronically lack sleep or their sleep is of poor quality, their health risks increase. They are more inclined to get high blood pressure, cardiovascular diseases, obesity, diabetes, and depression.

Sleep is a complicated dynamic process that impacts how well you function. Scientists are just starting to understand its complexities. This article discusses what regulates our need for sleep and what goes on in the brain when we sleep. 

The anatomy of sleep 

The hypothalamus is a tiny structure embedded deeply in the brain. It is made up of clusters of nerve cells. They function as control centers for both sleep and arousal. The suprachiasmatic nucleus (SCN) is located in the hypothalamus. It processes information from the eyes regarding light exposure and controls your behavioral rhythms. People who have sustained damage to their SCN will sleep intermittently during the day because they cannot synchronize their circadian rhythms with the earth’s natural day-night cycle. Interestingly, the majority of blind people are able to perceive some degree of light and therefore can adjust their sleep/wake cycle. 

The base at the back of the brain is where the brain stem is located. It communicates with the hypothalamus to control the sleep/wake transitions. The brain stem is made up of several structures: the midbrain, medulla, and pons. Certain hypothalamic and brain stem cells promote sleep by producing a neural chemical called GABA, which slows down the activity in the hypothalamic and brain stem arousal centers. Parts of the brain stem (the medulla and pons) affect REM sleep by sending signals to relax the muscles that enable limb movements and body posture so that we don’t physically react to our dreams. 

The thalamus relays the information coming from our five senses to our cerebral cortex. This is the part of the brain in charge of interpreting and processing the information into short- or long-term memories. The thalamus is quiet throughout most stages of sleep, allowing us to ignore the outer world. But when you’re in REM sleep, the thalamus is busy sending sounds, images, and other sensations to the cerebral cortex, which is what makes up your dreams. 

The pineal gland, which is located between the two hemispheres of the brain, gets a signal from the SCN to increase production of melatonin, the hormone that enables sleep once it gets dark. Blind people who cannot perceive light and are unable to naturally coordinate their sleep/wake cycle can take small amounts of a melatonin supplement at the same time every day to maintain a consistent sleep pattern. Biologists think that the highs and lows of melatonin production over time are essential for synchronizing our circadian rhythm to the natural cycle of daylight and darkness. 

The basal forebrain is located towards the front on the underside of the brain and also helps the sleep/wake cycle. At the same time, part of the midbrain functions as an arousal mechanism. The basal forebrain and other parts of the brain release a chemical called adenosine which also promotes sleep drive. Consumption of caffeine will block the activities of adenosine and therefore keep you awake. 

The amygdala (located in the frontal area of the temporal lobe) is responsible for perceiving and processing emotions. The amygdala can become very active during REM sleep. 

The stages of sleep

There are two types of sleep that we all experience: rapid eye movements (REM) and nonREM sleep. There are three stages in nonREM sleep, each linked to specific neuronal activity and certain brain waves. We all experience several cycles of both REM and nREM sleep throughout the night. As we approach the morning hours we experience longer and deeper periods of REM sleep. 

nREM sleep

Stage 1 

This stage occurs during the transition between wakefulness and sleep. This only lasts a few minutes of fairly light sleep. During this time certain bodily functions, such as breathing, heartbeat, and eye movements, slow down. Our muscles begin to relax although we may experience some twitching. Our brain waves also slow down.

Stage 2

This is when we sleep lightly right before falling into  deeper sleep. Our breathing and heartbeat slows down even further, while the muscles continue to relax. We experience a drop in the body temperature and the eyes stop moving altogether. The brain waves slow down overall, but there are short intermittent barrages of electrical activity. We spend more time in stage 2 than in stages 1 or 3. 

Stage 3

This is when we are in deep sleep, which is necessary for feeling refreshed when we wake up in the morning. Stage 3 periods last longer during the first half of the night. It’s in stage 3 that our breathing and heartbeat slow down to their lowest levels. Brain waves get even slower and the muscles become so relaxed that it may be difficult to wake us up. 

Rapid eye movements (REM) sleep

Approximately 90 minutes after falling asleep we enter REM sleep. During this stage our closed eyes are rapidly moving from side to side. The brain waves will resemble mixed frequency activity that occurs when we’re awake. Our breathing will speed up and become erratic, while our blood pressure and heart rate will rise to almost waking levels. The majority of dreaming happens during REM sleep, although some does occur during nonREM sleep. The muscles in our limbs literally become paralyzed, but only temporarily. This keeps us from acting out the events in our dreams. When we get older, we spend less time in REM sleep. The consolidation of memories most likely requires a combination of REM and nREM sleep.

Sleep regulation mechanisms 

There are two biological mechanisms - homeostasis and circadian rhythm - responsible for regulating when we’re awake and when we sleep:

The circadian rhythm

This mechanism controls a broad range of bodily functions including the fluctuations in levels of wakefulness, the release of hormones, the metabolism, and body temperature. Your circadian rhythm controls when you sleep and when you awaken in the morning. Your biological clock syncs with cues in the environment that indicate what time of day it is, such as temperature and light, however your circadian rhythm keeps going even when there are no cues. 

Sleep-wake homeostasis 

This biological mechanism monitors your body’s need for sleep. The drive towards homeostatic sleep tells the body that it needs to sleep after a certain period of time. It also regulates the intensity of sleep. This drive to get some sleep increases every hour you remain awake and will cause you to sleep longer and more deeply after you are deprived of sleep for a period of time. 

Other factors that affect sleep-wake cycles

Other factors that affect your body’s sleep-wake needs are medications, health conditions, what and how much you eat and/or drink, stress, and your sleeping environment. But what affects you most is how much light you’re being exposed to. The retinas in your eyes contain special cells that process light and inform your brain whether it’s day or night. These cells have the power to advance or slow down our sleep-wake cycle. Being exposed to light makes it hard to fall asleep as well as return to sleep when we wake up. 

People working night shifts often find it difficult to fall asleep when they finally go to bed. They also have problems remaining awake and alert at work due to the disruption of their inner sleep-wake cycle. When people fly and land in a different time zone, they often get jet lag because their circadian rhythms are no longer synchronized with the actual clock.

How much sleep do people really need?

There is no magical number of sleep hours we all need to function well. The need for sleep also changes as we age, along with our sleep patterns. And interestingly, not all people have the same need for sleep at the same age. We are all different. Infants can sleep up to 18 hours a day to spur growth and brain development. Schoolchildren and adolescents need approximately 9.5 hours of sleep a night on average. In adulthood we need 7-9 hours of sleep a night. But after we reach 60 our sleep tends to get lighter, shorter, and is often interrupted by waking up several times throughout the night. Furthermore, the elderly often use medications that can disrupt their sleep.

Generally, people are not getting the right amount of sleep because of longer hours at work and easier access to entertainment and any number of activities 24 hours a day. 

People who don’t get enough sleep during the week often plan on catching up over the weekend. But sleeping in and taking naps at weekends may not sufficiently restore them, depending on their level of sleep deprivation. 

Dreaming

We all dream. The average person spends approximately 2 hours every night dreaming, but we rarely remember our dreams. No one knows why we dream or what purpose dreams fulfill, but they may be helping us to process our emotions. Events that occurred that day often come to mind when we sleep, and people who are anxious or stressed out are more likely to have scary dreams. We can dream throughout the whole night, but we experience our most vivid dreams during REM sleep. Some people have colorful dreams, while others dream in black and white. 

Neurotransmitters, genes and sleep

How chemical signals affect sleep

Clusters of sleep-inducing neurons located in various areas of the brain start becoming more active at bedtime. Nerve-signaling chemicals in the brain called neurotransmitters will slow the activity of certain cells responsible for relaxation or arousal. GABA is the neurotransmitter associated with muscle relaxation, sedation, and sleep. The neurotransmitters orexin (a.k.a. hypocretin) and norepinephrine are responsible for keeping certain areas of the brain active during waking hours. Other neurotransmitters involved in wakefulness and sleep include serotonin, acetylcholine, adrenaline, cortisol, histamine. 

How genes affect sleep

Genes are believed to be a determining factor in how much sleep each of us needs. Scientists have pinpointed specific genes that affect sleep and that may be responsible for sleep disorders. These are the genes that direct neuronal excitability as well as “clock genes” (i.e. Per, Tim, and Cry) that affect our circadian rhythm and when we sleep. Research on genomes has identified specific spots on a number of chromosomes that raise our propensity for sleep disorders. Different genes seem to be responsible for different sleep disorders, such as narcolepsy, restless leg syndrome, and advanced familial sleep-phase disorder. 

Certain genes active in the cerebral cortex as well as other areas of the brain modify their level of expression between sleep-wake cycles. Scientific studies on worms, fruit flies, and zebra fish are helping scientists pinpoint the genetic variants and molecular mechanisms in normal sleep patterns and sleep disorders. Further research should help our understanding of inherited (genetic) sleep patterns and the health risks of circadian rhythm disruption and sleep disorders. 

Diagnostic tests for sleep disorders

If you suspect that you have a sleep disorder, your physician may schedule you for a polysomnogram or another diagnostic test to determine the cause of your sleep problems. A polysomnogram usually requires the patient to spend a night at a sleep lab or center. Specialized equipment will record your oxygen levels, breathing, limb and eye movements, brain waves, and heart rate throughout the night. You will also be audio and video recorded. The collected data will help determine whether you are reaching each stage of sleep and proceeding normally through all of them. The results will be used to help the physician decide if further tests are needed or form an effective treatment plan.

Smart technology to monitor sleep

Millions of people around the world are using wearable technology including smart watches, headbands, and bracelets as well as bedside monitors and smartphone apps to collect information about their sleep. Smart technology can make visual and sound recordings of your movements and sounds during sleep. It can also track the hours slept and monitor respiration and heartbeat. With the help of another app data gathered on certain devices can be synced with a tablet or smartphone and/or uploaded to a PC. Other types of technological devices and apps can make white noise, produce gentle vibrations, and stimulate the natural production of melatonin to help us fall asleep or wake up. 

Tips to help you sleep soundly through the night

To maintain good health, it’s important that you get sufficient sleep. Here are some tips to help you sleep better: 

• Schedule set times for going to bed every night and waking up in the morning. Do not deviate from the set schedule. 
• Get 20-30 minutes of exercise every day, but do not exercise during the 3 hours before bedtime.
• Avoid tobacco products and caffeine in the late afternoon as well as alcoholic beverages before bedtime.
• Do something relaxing before bedtime, perhaps reading, taking a nice warm bath, meditating, or something else that you find relaxing.
• Set your bedroom up to be conducive to sleep. It should be quiet, dark, and kept at a comfortable temperature. Do not use a computer or watch TV as you’re approaching bedtime and it would be even better if you didn’t have these devices in your bedroom at all.
• If you can’t get to sleep, do not lie in bed awake. Get up and do something that might relax you, like reading or listening to music.
• If you continue to have sleep problems or feel more tired than usual during daytime, see a doctor. There are effective treatments for most sleep disorders. 

Scientific research offers hope

Research continues to illuminate how sleep is regulated and what its functions are. One of the main foci of scientists is to gain understanding of the risks of chronic sleep deprivation and the connection between sleep and medical conditions. People who continuously lack sleep are more prone to being overweight, infections, certain cancers, cardiovascular diseases, and strokes than people who regularly get sufficient sleep. 

Disturbances of the sleep pattern are prevalent among those with Parkinson’s and Alzheimer’s disease, both of which are neurological disorders related to age. Much is unknown about the link between sleep and these medical conditions. Which came first, the chicken or the egg? Does lack of sleep cause certain medical conditions, or do these diseases cause difficulty sleeping? These and many more questions about sleep continue to be the subject of research being conducted now.