Chronobiology

If you’ve been keeping up with recent research, you’re probably aware of the negative effects of blue light from electronic devices on sleep. More and more we are being warned to put down the smartphone and power down the laptop at night. The link between low melatonin levels and blue light is well established. However, blue light may be affecting us in more ways than we previously realized.

Light, Melatonin and Sleep

Throughout human history, people have gone to bed shortly after dark. Our bodies are biochemically primed for this behavior. When our retinas stop sensing light, they send a message to the suprachiasmatic nucleus (SCN) of the hypothalamus. This part of the brain functions as the timer for our internal clocks, setting off a cascade of events that prepare us for sleep. One of the main ways this happens is through the release of melatonin in the pineal gland. Conversely, in the early morning hours when light first appears, melatonin production is shut down. At this time, our bodies switch to making cortisol and other hormones that increase alertness.

Realistically, a day in modern life does not end at sundown. We are exposed to artificial lighting from a variety of sources including light bulbs, televisions, computers and smartphones. Although even dim artificial light can affect melatonin production, it appears that the effects of blue light, such as that emitted from devices like tablets and smartphones, may be even greater.

More Than Melatonin: The Problem With Blue Light

Blue light is constantly present in our world. How many people go to bed with an e-reader, tablet, or smartphone in hand? This can have an immense impact on how long it takes us to fall asleep, as well as the quality of our sleep. Human eyes have special wavelength-sensitive photoreceptors that sense blue light, which appear to shut down melatonin production more effectively than other kinds of receptors in the eye.

Mice also have a circadian rhythm, although it is the opposite of ours. Mice are nocturnal and sleep mainly throughout the day, which means they are primed to produce melatonin and fall asleep in the presence of light. Although they have a very different reaction to light than humans, they have the same receptors for blue light in their retinas. When exposed to light, mice in a recent experiment became sleepy as expected. However, when exposed solely to blue light, these mice became more active and took much longer to fall asleep. This did not occur in mice who were bred to lack the blue-light photoreceptors.

Interestingly, blue light appeared to delay their sleep as much as it does in humans, having more of an effect than green and violet light. Scientists behind these studies believe this is because blue light may also increase levels of stress hormones like cortisol. These hormones are cues for wakefulness, explaining the wakening effects of blue light on both diurnal and nocturnal animals.

Blue Light: A Possible Cause of Circadian Rhythm Disruption

While blue light is present in sunlight, it is present in far lower amounts than that seen in electronic devices. Sunlight contains the full spectrum of light, which is why it can be refracted into a rainbow. On the other hand, most devices contain almost exclusively blue wavelengths of light. Given that blue light reduces melatonin production and may even increase cortisol levels, this has immense implications for human health.

Sleep disorders are on the rise in the United States; increasing blue light levels in our environment that inhibit melatonin production and increase stress hormone levels may be part of the cause. However, many people are tied to their devices. We use smartphones and tablets to perform work, to communicate with friends, even to read a book. Can modern people find a balance that allows them to get the sleep they need? This is the question for chronobiology researchers and other scientists who study circadian rhythm.

•  

Benjamin Franklin once observed, “Early to bed and early to rise makes a man healthy, wealthy and wise.” However, in reality, it may not be so simple. While some people are up with dawn and yawning by dusk, others come alive at night, yet struggle to function in the mornings. New research has found that whether you are a “morning lark” or a “night owl” is determined by your genes. How you manage your chronotype and your lifestyle can have far-reaching effects on your health.

What Are Chronotypes?

Your chronotype is generally set by your sleep midpoint. For instance, a person who naturally goes to sleep at midnight and wakes up at 8 a.m. will have a midpoint of 4 a.m., making them a natural night owl. People who go to sleep early and wake early naturally are morning larks.

Our internal clocks shift slightly during our lives. Most small children are naturally morning larks, as many bleary-eyed parents have observed. However, this schedule has shifted by adolescence. Adolescents and young adults naturally lean toward a later bedtime and sleeping in well past dawn. On the other hand, the elderly move back to an early bedtime. However, there is variation even within this general pattern. For example, some toddlers are consistently up at 4 a.m. while others “sleep in” until 7 a.m.

Society has classically viewed night owls as lazy. These people struggle to get up and off to work or school early in the morning and are often not on the job during their most productive hours of the day. This may make them appear lazier, when in fact they are suffering from a phenomenon known as social jet lag, or a mismatch between outside obligations and internal rhythms. New research suggests that the difference between night owls and morning larks is not based on motivation and morality, but rather on genetics.

What Determines Your Chronotype?

Scientists have discovered that different chronotypes have measurably different levels of proteins associated with the circadian rhythm and even different variants of genes associated with the sleep-wake cycle. In a recent chronobiology study, a group of people with varying chronotypes were tested for salivary levels of melatonin and expression of the circadian rhythm genes Per1, Per2, and Rev-erbα. Researchers found that night owls had later expression of melatonin than morning larks, which means these people were biochemically primed to sleep and wake at different times. Considering that a person’s amount and quality of sleep can have immense effects on both lifestyle and health, this can affect a natural night owl’s life in a variety of ways.

Marching to the Beat of Your Internal Clock

Ideally, people would be able to work and go to school at the times when they are primed to be productive. They would also be able to sleep when they are biologically intended to do so. However, this is not the case for many modern people. Many night owls are crawling out of bed in the morning far before their sleep cycles have completed; many morning larks are forced to be awake past their natural bedtime for various obligations. The result is not just a society of sleepy people, but one of unhealthy people. Living out-of-sync with your chronotype can have a variety of health consequences.

The Health Risks of Ignoring Your Circadian Rhythm

What, exactly, happens when people are forced to live out of sync with their chronotype? As researcher Frank Scheer noted in the Proceedings of the National Academy of Sciences, this can cause “a decrease in leptin, increase in glucose and insulin, increase in mean arterial blood pressure and reduced sleep efficiency.” This leads to an increased risk of metabolic syndrome, cardiovascular disease and a variety of health disorders.

However, night owls do not necessarily need to suffer from diseases associated with circadian rhythm disorders. When allowed to change their schedule to match their natural sleep-wake cycles, people suffer no additional health risks. Being a night owl is not in itself unhealthy unless you try to act like a morning lark.

Our work and other activities were once limited to the hours when there is natural light. However, modern technology and artificial lighting allow us to be productive at any time. Allowing night owls and morning larks to work and learn when they are primed to be so may not just create a more productive workforce, but a healthier one as well.

•  

A precise balance of salts in the body is important for a variety of biological functions, from nerve activity to fluid balance. With this in mind, it should come as no surprise that salts are also involved in maintaining a healthy sleep-wake cycle. However, new science reveals that salts appear to play a much bigger role than expected, acting as part of the regulation system of our circadian rhythm.

Electrolyte Balance and Brain Function

When ingested and absorbed into our body fluids, salts such as sodium and potassium become ions. These ions, also called electrolytes, are key parts of many biochemical reactions. They are also conductors of the electrical currents needed for muscle activity, neuron signaling and more. Salt balance is important to health. The role of salt in brain signaling is one reason the body works hard to keep sodium and potassium levels within a very narrow range.

When our neurons receive an impulse, sodium channels along the axon, which looks like a long open tail. Salt flows into the cell quickly, causing the electrical balance to change. This creates a current that is passed down the axon, opening sodium channels along the way. This current is passed to another cell, which repeats the process until the message has reached its target location. The majority of signals in the brain are transmitted using this process. Many disorders that affect both sleep and cognitive function, such as schizophrenia, have been associated with aberrant or faulty signaling of sodium channels.

Salts and Your Sleep-Wake Cycle

Because the brain is dependent on sodium and potassium balance to send and receive information, electrolyte imbalances can have a variety of effects, including disruption of the sleep-wake cycle. When people have even mild imbalances of salts, they will suffer from changes in their sleep patterns, fatigue, fuzzy thinking and even dementia. These are all symptoms we associate with disrupted sleep-wake cycles as well. Are these coincidental side effects, or could salts actually function as regulators of the circadian rhythm? This is what a set of Danish researchers set out to discover.

Salts, Neuromodulators and Sleep

Scientists know that neuromodulators such as cortisol and melatonin play a huge role in regulating the sleep-wake cycle. These neuromodulators affect salt balance by opening sodium channels as a means of signaling. Salts appear to act independently of these hormones, as they are controlled by non-hormonal means. While the relationship between neuromodulators and salts is still a subject of study and debate, salt appears to be an independent variable affecting sleep. When salts were injected into the brains of mice, their wakefulness, or lack thereof, changed in a predictable way. Researchers were able to put a mouse to sleep, or wake them, simply by adding salts. The relationship between neuromodulators and salt balance may actually be the mechanism by which they cause sleepiness or wakefulness.

This may seem far-fetched, but it is not an unusual way to control neural activity. The activity of sodium channels is an important regulator of many brain processes, so many hormones and pharmaceuticals work on cells by opening or closing these channels. It is possible that changing salt levels are an important way that the brain controls the sleep-wake cycle.

The Future of Sleep Disorder Treatment?

Obviously, people with sleep disorders cannot be treated by a shot of salts into the brain as the mice in the study were. However, there may be other applications for these findings. Many drugs act by changing the activity of salt in the brain. For example, many epilepsy drugs block the activity of voltage-gated sodium channels, which transmit most neural signaling. In short, people who suffer from circadian disorders may soon have a medication to help them regulate their sleep-wake cycle if other research supports the conclusion of this study.

While no one should begin eating more salt to treat a circadian disorder, this new discovery is nonetheless promising. Understanding how different variables such as salt balance affect the brain, and by extension, our sleep-wake cycles will allow us to better understand how circadian disorders develop.  This can, in turn, lead to more effective treatments for people who suffer from sleep disorders.

•