In this lesson, you will learn about the science of sleep. It may seem like not a whole lot is going on while you sleep, but this is wrong. In reality, your brain is quite active when you sleep. Further, your body actively prepares you for sleep as the day passes.

Understanding the science of sleep may help you better understand your own sleep patterns.

What is Sleep?

Sleep is a necessary biological process. Without sleep, you could not function; although, it is not exactly clear to scientists why we need sleep.

You could describe sleep as a period of physical inactivity and reduced consciousness. However, during sleep, your brain is still active. Your brain cycles through two major stages of sleep: rapid eye movement (REM) sleep and non-rapid eye movement (NREM) sleep.

REM sleep is when you have vivid dreams. Your brain activity is similar to or more intense than when awake. Interestingly, your eyes move quickly back and forth, but your limbs can’t move during this stage. This prevents you from physically acting out your dreams. Also, your brain waves, breathing, and heart rate are variable during this time (similar to when you’re awake).

Sleepwalking occurs when a person is in REM sleep, but their body does not immobilize their limbs.

During NREM sleep, you don’t usually dream and your eyes are still. Your brain activity becomes coordinated and regular (unlike REM sleep). Your brain waves, breathing and heart rate become slower and slower. As you progress through the NREM stage, you fall into a deep sleep.

While you sleep, your brain cycles through these stages. Throughout the night, you go from being awake to REM sleep to NREM sleep several times. The above graph shows how you may go through each stage several times each night. As you can see, you spend the most time in NREM sleep; this is the part of sleep that makes you feel most restored.

Why We Need Sleep

Scientists still speculate exactly why we need sleep. The purpose of sleep is a vague question-mark in the science of sleep.

You’ve probably experienced a sleepless night that has left you feeling horrible the next day. This makes it quite clear that you need sleep to feel well. Getting enough sleep can help you to function at your best. When you’re well rested, you’re more attentive, energetic, and resilient. If you didn’t need sleep, there would not be such an intense biological drive to sleep.

Here are some theories:

Some theories suggest that sleep allows us to conserve energy. While you sleep your metabolism, body temperature, and demand for energy reduces.

Another theory is that sleep helps our bodies to restore themselves. This theory is backed by a lot of evidence. For example, in studies where animals are kept awake, they die in a few weeks. Their immune system becomes severely impaired. Additionally, several restorative processes occur mostly during sleep. This includes tissue repair, muscle growth, protein synthesis, and growth hormone release. Important note: these processes are important, especially for people with chronic pain.

A third significant theory relates to brain development and organization. Sleep may have a role in shaping your brain. For babies, sleep is vital. Sleeping babies spend most of their time in REM sleep. Their brains develop as they dream. During sleep, neural connections and pathways are reinforced.

In adulthood, your brain still organizes itself while you sleep. Interestingly, the amount and quality of sleep you get impacts learning and memory. It’s as if your brain is filing away memories as you snooze. This may work the same way that a baby’s brain develops. As you sleep, your brain reinforces the neural connections that form the memories of what happened during the day. This is called memory consolidation: the process of “locking in” a memory.

Conversely, poor sleep can negatively impact your mood. A negative mood can affect learning and recall of memories. In other words, if you don’t get a good sleep, you may be grumpy and forgetful!

Another function of sleep is to flush out the metabolic by-products of brain activity. These build up during the day. At night, your brain gets rid of them. If you don’t get a good sleep, these by-products stick around and can make it difficult for your brain to function the next day. We’ll talk more about these by-products in the next section!

Circadian Rhythm

This aspect of the science of sleep is so interesting! We have an internal 24-hour clock that regulates our body’s processes. It’s called your circadian rhythm. It manages several functions of the body, including sleep. This rhythm is heavily reliant on your environment; in fact, it is sensitive to light. It helps you stay awake when it’s light out and become sleepy when the sun goes down.

The circadian rhythm controls the sleep-wake cycle. Light tells your brain that you should be alert and active. As time passes and the day gets darker, you produce a hormone called melatonin. Melatonin has a role in triggering and sustaining sleep.

As the hours pass during the day, your sleep drive increases. Another compound that tells your body that it’s time to sleep is adenosine. Adenosine is produced as a result of energy-use by your cells. So, the longer your cells are consuming energy, the more adenosine is made. By the end of the day, adenosine levels in the brain are at their highest, promoting sleep. Once you sleep, adenosine levels decrease.

These two systems work well together. Although your sleep drive steadily increases throughout the day, your circadian rhythm keeps you alert while it’s bright out. The message that you should stay awake gets more intense throughout the day to combat your increasing sleep drive. However, once it becomes darker and the message to stay alert decreases, the drive to sleep takes over. You become drowsy and you know it’s time for bed.

When you fall asleep, the message to stay alert continues to decrease while your sleep drive is high. At this point, staying asleep is easy. Then, after a few hours your sleep drive decreases, and you become easier to wake. Your internal clock takes over once again as it becomes brighter in the morning. This process usually allows you to sleep 7-8 hours each night.

These elements work in harmony. However, when they become out of sync, you can experience sleep disturbances.

Disruptions in Sleep

Although the consensus on the science of sleep is still not clear, the impacts of poor sleep are obvious. Some reasons why sleep might be disrupted are discussed below.

When your circadian rhythm and sleep-wake cycle become out of sync, you can have trouble sleeping. Several external and internal factors can affect your ability to sleep.

Caffeine, for example, works by blocking adenosine receptors in the brain. This tricks your brain into staying alert. In other words, it impacts your sleep drive. If you have caffeine before sleeping, it strongly affects the quality of your sleep. You experience less REM sleep and deep NREM sleep. Further, you may awaken more often during the night.

Your chosen sleep pattern can impact the quality of your sleep. For example, if you do shift work overnight or travel through different time zones, your internal clock and sleep-wake cycle will become misaligned. As your internal clock is cued by light, being awake when it’s dark out can make you extremely sleepy. Further, trying to sleep when it’s bright out can be a challenge

Medical conditions can also impact your sleep. For example, anxiety and chronic stress can prevent you from entering a deep sleep. Evolutionarily, we are programmed to stay awake or wake up easily in response to stressful or dangerous situations. So, if you are chronically stressed, you may not be able to get a restful sleep. Initially, it can be hard to fall asleep. When you do eventually fall asleep, you may have more REM sleep and less deep NREM sleep.

Another major factor that can disrupt your sleep is pain. Pain can make it hard to stay asleep for long enough to achieve deep sleep. In the next lesson, you will learn about the relationship between sleep and chronic pain.

Reflect 

Take a moment to reflect about your sleeping habits. Do you find it challenging to sleep? Does anything in this lesson stick out to you as something that might affect your sleep?

You may find it helpful to write down your thoughts. It will help you keep track of them as you go through the course.

To continue to the next lesson, take the quiz first.

References

Harvard Medical School. (n.d.). How sleep boosts your energy. Harvard Health Publishing. https://www.health.harvard.edu/healthbeat/how-sleep-boosts-your-energy

Harvard Medical School Division of Sleep Medicine. (2008). External Factors that Influence Sleep. Healthy Sleep. http://healthysleep.med.harvard.edu/healthy/science/how/external-factors

Harvard Medical School Division of Sleep Medicine. (2008). Natural Patterns of Sleep. Healthy Sleep. http://healthysleep.med.harvard.edu/healthy/science/what/sleep-patterns-rem-nrem

Harvard Medical School Division of Sleep Medicine. (2008). The Characteristics of Sleep. Healthy Sleep. http://healthysleep.med.harvard.edu/healthy/science/what/characteristics

Harvard Medical School Division of Sleep Medicine. (2008). Why Do We Sleep, Anyway? Healthy Sleep. http://healthysleep.med.harvard.edu/healthy/matters/benefits-of-sleep/why-do-we-sleep

Society for Endocrinology. (2018, March). Melatonin. You and Your Hormones. https://www.yourhormones.info/hormones/melatonin/

Suni, Eric. (2020, September 25). Circadian Rhythm. Sleep Foundation. https://www.sleepfoundation.org/circadian-rhythm