Understand Your Circadian Rhythm: How Light Affects Sleep

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In this article Ms. Smith describes how the appropriate circadian rhythm lighting can improve our sleep but also metabolism, thyroid function, mood, and stress response. WalaLight uses this same theory in the science behind the spectrum and intensity of light that the LED fixtures provide both day and night.

or centuries, humans slept in segments. They would go to bed around 9:00 p.m. or 10:00 p.m., sleep for three to four hours, and wake up after midnight for an hour or so. During that time they might pray, meditate, have sex, or even perform simple chores that didn’t require much illumination or skill. Then they would go back to sleep for another three to four hours, finally getting up around dawn. This strange sleep pattern wasn’t the result of insomnia, it was what’s known as biphasic sleep.

“Our sleep today — and by our I’m referring to people who live in North America and the Western world — is remarkably young. It’s an artificial product of modernity that arose around the late 19th, early 20th century,” says Roger Ekirch, a professor of history at Virginia Tech who was one of the first to publish on biphasic sleep. “That’s when the norm became consolidated or seamless sleep, to which we aspire — though not always successfully, of course — today. The transition was prolonged and erratic from segmented to consolidated sleep, but it generally occurred over the course of the 19th century with the first sleep that people took becoming elongated as the century unfolded.”

“If the light’s on, it’s day. If the light is off, it’s night, according to the brain.”

Artificial light was the cause of the transition — first through gas lighting and then the invention and widespread adoption of Thomas Edison’s incandescent light bulb. The technology made it possible (as we experience all too often today) for people to stay up working or socializing until 11:00 p.m. or midnight. As a result, people wouldn’t awaken from their first sleep until 3:00 a.m. or 4:00 a.m., and by that time, Ekirch says, they might as well get up and start working. As a result, sleep became more compressed and waking up in the middle of it was a burden, not an asset.

This shift in sleep patterns is one of the clearest examples of how artificial light has influenced our circadian rhythm — the body’s internal clock that affects everything from sleep to metabolism. Humans evolved to depend on the sun as their only source of bright light, so for millennia the body’s 24-hour circadian rhythm was dictated by sunrise and sunset. Now, people can be exposed to bright light no matter what time it is, and our eyes are unable to distinguish whether light is from the sun or man-made. As a result, the body responds to bright light exposure at night in the same way it responds to daylight.

“If the light’s on, it’s day. If the light is off, it’s night, according to the brain,” says Steven Lockley, a neuroscientist at Brigham and Women’s Hospital and an associate professor of medicine at Harvard Medical School.

Every organ in the body abides by an internal clock, but the master pacemaker is a set of neurons in the brain known as the suprachiasmatic nucleus (SCN). Retina cells in the eye send light information to cells in the SCN, which causes genes in those cells to turn on. When the genes are turned on, the cells produce proteins that trigger a cascade of changes throughout the rest of the brain and body. Eventually, the protein levels in the SCN cells get high enough that they signal the genes to turn off, and the protein levels fall.

This whole cycle takes approximately 24 hours, and it is kicked off each morning with exposure to sunlight to synchronize the body with the environment. If no light is available, the body maintains a 24-hour clock, but it is no longer tied to light and dark.

“Light is this really strong time cue that’s used by the body to set the clock,” says Stuart Peirson, a neuroscience professor and circadian rhythm expert at Oxford University. “As a result, it means that when we’re exposing ourselves to artificial light we’re actually pushing our clock forward and backward inappropriately.”

Research suggests that our modern lifestyle has shifted human’s biological clocks by approximately two hours. For example, one study measured the circadian rhythms of people who were camping in the wilderness for a week and had no exposure to artificial light. In a natural environment, they were exposed to much more sunlight during the day and virtually no light at night. Their levels of the sleep hormone melatonin — a classic indicator of circadian rhythm — rose starting at dusk, peaked around midnight, and dropped off at sunrise. When the same people were back in the “real world,” their biological clocks were shifted by two hours, with the onset of melatonin occurring after dark and subsiding after they woke up in the morning.

“What the modern light environment does is cause us to go to sleep too late, shifting both our sleep and internal clock later than it would be in a natural life cycle,” says Lockley, who was not involved in the study.

Two aspects of modern life contribute to this shift: one is too much light at night, the other is too little light during the day. Humans are particularly sensitive to light during the night since this is the time when they should be asleep. Nighttime light can result in the suppression of melatonin production and a delay in the onset of sleep. The other disrupting factor is less light at midday. People today spend significantly more time indoors than outdoors, and while electric light is strong enough to trick our eyes into thinking it’s daytime, light bulbs and screens aren’t actually as powerful as the sun.

Our circadian rhythms start to respond to light at around 100 lux, which is about the brightness of a dim room. Most office buildings are designed to have a light intensity of 300 to 500 lux, but that’s nothing compared to a bright sunny day with a light intensity 100-times that. Even on a cloudy day, the light intensity can be 1,000 lux.

Besides making it harder to fall asleep at night or get up in the morning, what are the ramifications of an altered biological clock? Circadian rhythms impact not only sleep but also metabolism, thyroid function, mood, and stress response. As a result, completely swapping day and night can cause a lot of health problems, and nighttime shift workers, such as medical professionals, factory workers, and flight attendants, have a higher risk of breast and prostate cancers, obesity, diabetes, and depression. For most people, though, the health impacts of adjusting the circadian rhythm by an hour or two are less clear, although there’s some evidence it can lead to weight gain and insomnia.

In 2017, Mariana Figueiro, director of the Lighting Research Center at Rensselaer Polytechnic Institute, examined the effect of light on office workers and concluded those with bright morning light exposure — both daylight and strong electric lighting— fell asleep easier at night and had increased sleep quality overall. When the men and women were subjected to bright light all day, either from electric light or daylight, they had reduced depression, too.

Two aspects of the modern lifestyle contribute to this shift: one is too much light at night, the other is too little light during the day.

“In all the studies, we’ve shown that when you increase the amount of light during the day, whether it’s by going outside, whether it’s increasing your electric lighting output,” she says, “you sleep better at night, you’re less depressed, you’re more alert. All of these things have an effect on people.”

Reducing light at night is also important to keep your internal clock on time. Instead of using overhead lights, switch on a lamp with a weaker light bulb. Using dark mode on phone and computer screens can help, too — not necessarily because of the infamous blue light, but because it will decrease the overall intensity of the light.

Try not to worry too much about burning the midnight oil, though. Even though electricity has made nighttime activity more common, it doesn’t mean our ancestors were always in bed at sunset and up at sunrise. Ekirch says that in the early modern era, people would perform chores by moonlight and starlight, go to the local tavern at night to socialize, or stay up late telling stories around a fire. “Darkness did not dramatically bring either work or entertainment to an abrupt end, far from it,” he says. “Nighttime was much more active, much more vibrant than the vast majority of people today would assume.”

Additional reporting by Allie Volpe

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