While we all have differences that make us unique, the need to sleep is something that all humans share with each other as well as with most other species on Earth. In this article, I will offer insight into the biology that connects us to our environment by guiding how we fall asleep and wake up using cues from the sun and our planet. It is important to point out that how we sleep changes as we grow. Newborn babies usually sleep a lot throughout the day but are awake at night. This is likely because something called circadian rhythms are not fully formed yet. Circadian rhythms are changes in our bodies, minds, and behaviors that follow a cycle which is very close to 24-hours. Circadian rhythms are so important that they are one of the earliest biological systems that emerges in babies. These rhythms usually develop by the time babies reach two to three months of age. Once important physical changes in an infant’s body begin to follow a 24-hour cycle more closely, they begin to sleep more regularly during the night. They also sleep for less time over a 24-hour period. As children age, they usually take fewer naps and eventually settle into a pattern where they stay awake during the day and sleep throughout the night.
Circadian rhythms occur in almost every cell and tissue of our bodies. These rhythms are governed by our genes and allow us to time when we are awake and asleep in response to sunlight and darkness. More precisely, there is a set of genes, called “clock genes,” that turn on and off over the course of the day and night. Once these genes are turned on, or expressed, they produce proteins and a cascade of internal events occurs that affects our entire beings. These events not only include putting us to sleep but also producing hormones and regulating our metabolisms. To allow these events to be cyclic, some clock genes make proteins that work to express other clock genes. These, in turn, make proteins that build up in our systems over the day and eventually can turn off their own genes. Then the cycle of switching clock genes on and off begins again. This biological clock can keep time even in the absence of sunlight. Importantly, even though biological clocks continue to run without light and dark changes, when our eyes see sunlight, wake-promoting clock genes can turn on rapidly. Therefore, it is important to align our sleeping, eating, and exercising to coincide with the cues we receive from our environment.
Similar to many of the important processes that our bodies perform, there are checks and balances that help control sleeping and waking periods. For instance, our biological clocks are helped by another process called sleep/wake homeostasis. Basically, this means that the longer we stay awake, the more our bodies realize the need to put us back to sleep. Also, just like our clock genes, many other signals that help wake us up or put us to sleep are sent by our bodies in response to signals we receive from our environment. Namely, when we see sunlight, a signal is sent to our brains that tells us to stop making something called melatonin. Melatonin works to help us go to sleep by sending sleep-promoting signals from the brain to different parts of the body. Since these sleep-promoting signals stop being sent in response to sunlight, this helps explain why looking at devices that emit artificial light can make it harder to fall asleep at night. Some wavelengths of artificial light, like the sun, can send signals to our brains that stop us from making melatonin. Once we stop producing melatonin, it can no longer work to help us fall asleep. It also can’t perform its other important duties which include fighting infections and promoting immunity.
Melatonin is produced from serotonin by a series of chemical reactions. Serotonin is important for helping us control our moods and stay focused. Light exposure, especially early in the morning, may help increase the amount of serotonin in our brains. This can then allow for more melatonin to be made when the sun begins to set. This is why people think that experiencing bright light early in the morning can help us fall asleep more easily at night. Exposure to light also stimulates adrenaline and cortisol production. Both of these hormones impact our ability to convert food into energy. They also influence how we respond to different types of stress. Circadian rhythms are not only influenced by light but also by temperature and seasonal changes. Because many processes in our bodies are closely regulated by circadian rhythms, timing is indeed everything. The time of day when we expose our eyes to light, eat and exercise can all influence how we sleep and how well our metabolisms work. In the end, circadian rhythms evolved so that we could live healthier and more comfortably in our earthly environment.
Sometimes, however, the genetic changes that make us unique can change how our bodies respond to the cues we receive from the environment. For example, the genetic changes that contribute to a child having a developmental or intellectual disability can also cause our brains to signal for us to sleep at the wrong time. A well-described illustration of this is for people who have Smith-Magenis syndrome. Individuals with this syndrome produce the most amounts of melatonin early in the morning instead of at night. This happens because a gene located in the part of the chromosome that is deleted in individuals with Smith-Magenis syndrome also provides instructions for making a protein that helps turn on and off several genes involved in circadian rhythms. Researchers are finding that changes in many genes that aren’t known to cause other conditions could also alter our circadian rhythms. We are still learning what all of these changes are and how they work. But as we continue finding the links joining the changes in our genes with the changes in when people sleep, we are learning how to better treat these problems. However, even without special medical treatments, when we understand at what time our own personal clocks are set, then we can better identify ways to use this information for improving our health.
It also valuable to note that – just like circadian rhythms – the physiology of sleep, or how our brain moves through different sleep stages, also changes as we grow from infants into children. Sleep physiology continues to change as we grow into adolescents and adults. In fact, sleep patterns even change as we age from middle-age into our elderly years. But before explaining how sleep patterns change as our bodies change, it may be useful to define the stages of sleep.
Sleep can be broken up into four stages that are defined by measuring brain waves using electrodes. The stages of sleep include light sleep (N1 or Stage 1), moderately deep sleep (N2 or Stage 2), very deep sleep (N3 or Stage 3) and dream sleep (REM or rapid eye movement sleep stage). While sleep is staged using devices that capture brain waves, different sleep stages also have physical characteristics that can be observed simply by looking at someone who is sleeping. Light sleep can be recognized when someone’s breathing slows to a regular pace. The second stage of sleep can be recognized when someone’s heart rate slows down, and their body temperature drops. In the deepest stage of sleep, our bodies reach a point of extreme relaxation. This is why it is very difficult to wake someone up when they are in Stage 3 sleep. These first three stages are collectively called non-REM sleep. REM sleep can be recognized when someone’s breathing is not regular, and their eyes are quickly moving under their lids. Our bodies also try their best to keep our muscles paralyzed during this stage of sleep, so we don’t act out our dreams. There are many theories about what happens in our brains and bodies during different stages of sleep. It is thought that each stage of sleep may allow for many of the reasons why we sleep to occur – like those presented in the previous article in this sleep series. It is also possible that different parts of our brains do different things during different stages of sleep. Even though we may not know all of the reasons for the different sleep stages, it is becoming clear that each stage of sleep serves a purpose for ultimately renewing our minds and bodies.
Now that sleep physiology has been defined, the way these patterns change throughout a person’s life can be more easily described. Once someone’s circadian rhythm starts to settle into a regular pattern, sleep stages tend to do the same. Usually, we move from being awake into the first phase of light sleep. Light sleep tends to last for only a short period of time, generally just a few minutes. Soon our sleep becomes deeper and we move into the second stage of sleep. This stage of sleep lasts much longer than the first stage. We then move from the second sleep stage into our deepest sleep. After we spend some time in the third stage of sleep, we begin dreaming. Our brains then repeat the cycle of moving through each stage of sleep. As we move through more and more complete cycles during the night, we spend less time in our deepest stage of sleep. Instead of deep sleep, we spend time dreaming and sometimes our brains can move from Stage 2 sleep straight into the dreaming stage. Newborn babies may sleep as much as 18 hours during a single 24-hour period, and they spend almost half of that time in REM sleep. They may also not move through every stage of sleep but instead go from being awake directly into the dreaming stage.
As people get older, they begin to sleep for less time during a 24-hour period. Most adults need only seven to eight hours of good quality sleep to feel rested in the morning. Adults also spend less of their sleep time in the deepest stage of sleep and more in the second stage of sleep. Research has also shown that changes in our genetic material can also alter our sleep stages. For example, genetic changes have been found to relate to sleep problems like narcolepsy and insomnia. We are still learning about how our genes can change sleep physiology, why sleep changes as we age and grow, and how each stage of sleep is important to our overall well-being.
Even though we may not understand why all of these biological phenomena occur, knowing how circadian rhythms and sleep patterns typically develop across the lifespan allows us to notice when these are atypical. This is especially interesting since these biological systems are established months, and sometimes years before many developmental or intellectual conditions can be diagnosed. It is possible that atypical patterns of circadian rhythms and sleep patterns are indicative of a child having a developmental or intellectual disability that could benefit from earlier interventions. One intervention may be to focus directly on finding better ways to improve sleep behaviors. For example, researchers have found that sleep problems relate to worse daytime symptoms in children with autism spectrum disorder (ASD). Furthermore, treating these sleep problems can improve daytime symptoms.
Symptoms of ASD could give rise to sleep problems. It’s also possible that the same genes affect both ASD symptoms and sleep problems. In fact, some of the genes involved in the development of our brains play the same roles in biology as genes that affect sleep physiology and the timing of sleep and wake. This is also true for our metabolisms. This suggests that the biology of sleep, neurodevelopment and metabolism could be connected. For those of us who research these connections, the hope is that this work will help us find better ways to treat sleep and circadian rhythm problems.
To summarize, although we know a considerable amount about the biology of how we sleep, there is still a lot to learn. The more we learn, the easier it will be to help people get better quality sleep. It’s possible that finding ways to get proper sleep during the early years of life has positive health impacts that extend into adulthood. Ultimately, finding ways to get better sleep at the right time should allow us to fully benefit from our biological connection with the natural world.
ABOUT THE AUTHOR:
Dr. Olivia Veatch is an assistant professor in the department of Psychiatry and Behavioral Sciences at the University of Kansas Medical Center. She has a Ph.D. in Human Genetics. Dr. Veatch’s research focuses on combining evidence from the laboratory with the power of computers to find ways that genetics can help inform healthcare. She is particularly interested in understanding the genetic changes that contribute to sleep problems in individuals with developmental or intellectual conditions.
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