The Complete Guide To Light And Sleep

Table of Contents
effects of light on sleep

Throughout the history of human evolution, we know that our ancestors depended on the sun as their only source of bright light. And before the invention of electricity, the goings on in their lives were largely dictated by sunrise and sunset. Not only did the availability or absence of light dictate what they could or could not do, but it also played a significant role in the human circadian rhythm

Our ancestors slept under cover of darkness and worked during daylight hours. And while many of us certainly know that darkness and sleep can and should go hand in hand, there’s far more to the relationship between light and sleep. 

How Does Light Affect Sleep?

Today we know that our circadian rhythms operate on a 24-hour cycle. Whereas darkness is a universal cue for sleep, “light is a signal for wakefulness. In fact, it is the primary influencer on our brain’s body clock,” Dr. Audrey Wells, MD, Founder of Super Sleep MD, tells Sleepopolis. 

And while light exposure in the morning can do good things for your body, mind, and circadian rhythm, Wells warns that timing is everything, as “light exposure in the evening affects your sleep quality and could make it difficult to fall asleep.” 

But it doesn’t end there. Light also affects melatonin production, sleep cycles and, as we mentioned earlier, circadian rhythms. 

Circadian Rhythms 

Light plays a significant role in regulating our sleep-wake cycle, aka our circadian rhythm. Exposure to light in the morning helps to synchronize our internal clock with the 24-hour day, while exposure to light at night can disrupt our sleep and make it difficult to fall asleep or stay asleep.

Circadian rhythm disorders are sleep-wake disorders that occur when there’s a misalignment between a person’s internal clock and the external light/dark cycles around them. 

Melatonin Production 

Melatonin has gained a lot of notoriety over the last decade or so. In the universal search for better or more sleep, people have elevated this humble hormone to rockstar status. And in some ways, it makes perfect sense. Often referred to as the sleep hormone, melatonin is produced by our bodies naturally. It regulates our circadian rhythm and plays a starring role in our sleep-wake cycles. Essentially, rising melatonin levels lead to drowsiness and, in turn, sleep.  

Digging a little deeper, we find that light profoundly impacts our ability to produce melatonin. While exposure to light puts the brakes on melatonin production, Dr. Robert Oexman, President and Founder at Somly, tells Sleepopolis “when we don’t have that signal from light, like in a dark room, for example, we will produce melatonin.” Oexman adds, “[melatonin production] happens even when we’re exposed to small amounts of light, and it’s not an on-and-off switch — a small amount of light won’t completely stop your melatonin production, but it will dysregulate its production.”

Oexman also notes that “melatonin production differs from person to person. Some people can be exposed to more light and still produce adequate amounts of melatonin, and for others, even a small amount of light can deregulate their melatonin production — there is no ‘one size fits all.'”

“Melatonin production differs from person to person. Some people can be exposed to more light and still produce adequate amounts of melatonin, and for others, even a small amount of light can deregulate their melatonin production — there is no ‘one size fits all.'”

– Dr. Robert Oexman, President and Founder at Somly

Sleep Cycles

Light can also impede smooth transitions between sleep cycles, and the result could be a fierce blow to your sleep quality. 

“When we transition from one sleep stage to another, we have a tendency to wake up, move around a bit, and go back to sleep,” says Oexman. “If there’s any type of light exposure during these transitions that could wake us up, whether it’s the light of the TV or the moonlight coming through the window, that can decrease the amount of melatonin that’s produced. And as more light hits the eyes, the more likely it will be a rough transition.”

Jet Lag 

Anyone who’s flown across the country or taken a vacation in another country could tell you their sleep cycle is a little wonky for a few days after the fact. Commonly known as jet lag, this sleep-wake disturbance happens when your internal circadian rhythm is out of sync with the local time zone of the place you traveled to. 

Your body may be telling you it’s 9 a.m. in New York (and time to be awake), but your travels have put you in Japan, where it’s 10 p.m. and time to sleep. When your exposure to light deviates from your body’s normal schedule, you may experience some of the more common symptoms of jet lag, including daytime sleepiness, fatigue, difficulty falling asleep, and irritability. There are ways to get your body back on track — it just takes some planning ahead.

Shift Work Disorder

Joseph Dzierzewski, Ph.D., vice president of research and scientific affairs for the National Sleep Foundation, tells Sleepopolis “light is the strongest factor when it comes to setting our body’s internal clock, which follows a natural 24-hour cycle, or circadian rhythm. Essentially, our brains are hard-wired to be alert and awake during daylight hours and less alert and fall asleep when it gets dark at night.”

Healthcare workers, law enforcement, and military personnel are often required to work night shifts, and as a result, they must sleep during the daytime. This runs counter to normal biological rhythms, and, unfortunately, it’s not without consequences. 

Shift work disorder is a circadian misalignment that profoundly impairs sleep. Shift workers are required to sleep when their circadian alertness is highest, which often translates to short and deeply fragmented sleep. As a result, the disorder is characterized by excessive sleepiness, insomnia, and “impairments in both sleep and wakefulness.” 

What Type of Light Affects Sleep?

Light can affect our sleep in both positive and negative ways. Timing matters — daylight can have a positive effect on sleep by regulating our circadian rhythm and promoting wakefulness during the day. It can also boost our quality of sleep and help us fall asleep faster — if we get the exposure early enough. On the other hand, late light exposure can disrupt the transition between sleep cycles, lead to frequent waking, and even reduce your time spent in slow wave sleep (the deeper sleep stages, which are the most restorative). 

But, of course, there are many wavelengths of light, including blue, red, green, white, and yellow, and each color of light affects our sleep in different ways.

Blue Light

Whereas bright light or daylight has a positive effect on your sleep, blue light can be quite harmful. While research has found that blue light can boost attention and alertness, improve your mood, and increase reaction times, research also shows blue light has a powerful suppressive effect on melatonin production (twice as long as green light) and can shift circadian rhythms by as much as three hours, delivering a powerful one-two punch to your sleep. 

If you’ve paid even a modicum of attention to health news, you know that blue light comes to us courtesy of our favorite things — the devices we tune into all day, every day. 

“The short-wavelength blue light emitted from electronic screens (smartphones, tablets, computers, TV, and eReaders) is particularly effective at signaling ‘wake’ to your brain,” says Wells. “If you are using these devices at night, your brain is getting mixed signals, and your sleep quality suffers.” According to Wells, the trouble with blue light often looks like “a difficult time waking up in the morning or daytime sleepiness.” 

While current studies are hard to come by, older research suggests that humans are sensitive to green light. The lion’s share of green light typically comes to us from the sun, but electronics like TVs, smartphones, etc., also emit green light. And while green light readily suppresses melatonin production, its suppressive effects are not as powerful as that of blue light. Older research also shows that red light has a negligible effect on sleep

Oexman notes that if there’s “any light at all, there is an opportunity to decrease melatonin production, but blue light tops the list of the light spectrums we don’t want at night.” He also warns anyone using blue light blockers or filtering out the blue light in any way shouldn’t be lulled into a false sense of security and take care not to “double down on other behaviors that can affect your sleep.” 

Should I Sleep in the Dark?

Your circadian clock is extremely sensitive to light, so a dark room (or sleep environment) is crucial to getting quality sleep. “A sleep-friendly environment is one that is dark, cool, and quiet,” says Dzierzewski, who also notes that recent evidence suggests even low levels of light during the night might interfere with sleep and can disrupt naturally occurring nighttime processes. 

To keep your sleep on track, Dzierzewski recommends “dimming the lights as you unwind in the evening and turning off screens (phones, tablets, televisions, and laptops) at least one hour before your usual bedtime.” He also says “blackout curtains and blinds are great ways to eliminate light pollution and remove outside light, creating a dark environment that’s primed for sleep.” 

Why Sleeping with Lights on Is Bad for Your Health

Not only can light impair your sleep, but a recent study out of Northwestern University found that exposure to even a moderate amount of light while you sleep could lead to higher heart rates and insulin resistance. 

If that’s not enough to convince you, research also shows that artificial light at night can be a risk factor for weight gain and obesity. Moreover, another study showed that artificial light at night might lead to prostate and breast cancer

How to Use Light to Wake Up

“Make it a habit to step outside each morning or afternoon — you’ll be surprised how just a little sunlight can help you feel awake, alert, and ready for the day. “

– Joseph Dzierzewski, Ph.D., vice president of research and scientific affairs for the National Sleep Foundation

“Exposure to daylight (full spectrum light from the sunshine) is a powerful way to feel more alert,” says Wells. “If you get sunlight exposure in the morning, it helps to anchor your circadian rhythm and improve both wake quality and sleep quality.” So as a best practice, it’s a good idea to get some light exposure when you wake up — the sooner, the better.

Dzierzewski doubles down on the advice. “When you wake up in the morning, nothing beats getting outside to enjoy the benefits of natural light,” he says. “Make it a habit to step outside each morning or afternoon — you’ll be surprised how just a little sunlight can help you feel awake, alert, and ready for the day.” 

He also suggests looking for more opportunities to incorporate light exposure into your daily routine. “If you start your day with a cup of coffee, make it a point to drink it outside — weather, climate, and safety permitting — to get a healthy dose of natural light. Treat your dog to an extra 10-minute walk or two in the afternoon. And if you exercise, try going for your daily walk or run outside instead of working out indoors.”

What Is Light Therapy Used for?

Also known as phototherapy, bright light therapy uses appropriately timed light exposure to reset disrupted circadian rhythms. Often used for treating insomnia and circadian rhythm sleep disorder, this type of therapy aims to gradually shift the individual back to a regular sleep schedule. The treatment itself is quite simple. It typically includes exposing the patient to a light box in the morning for anywhere from 10 to 60 minutes.

How to Sleep During the Day

Sleeping during the day isn’t the norm, but as a result of life in the 21st century and 24/7 access to everything, it’s a necessity for some occupations. And while you might think that your sleep drive after an 8-hour shift (sometimes longer) would be enough for you to catch your zzz’s with relative ease, the daylight outside your window (and other environmental factors, spouses, kids, and life in general) says it ain’t so. Those who need to sleep during the day must address the most pressing factor that could stymie their shut-eye, you guessed it — light.  

According to Oexman, “the approach to sleeping during the day and napping are completely different.” For night shift workers who have to sleep during the day, he first and foremost recommends an eye mask that completely blocks out the light. 

For those who aren’t on the night shift and choose to nap during the daytime (which he cautions against unless you sleep well at night), he doesn’t recommend using an eye mask to avoid slipping into deep sleep stages. 

Other suggestions for shift workers to manage circadian rhythms that are upside down include blackout curtains when it’s time to sleep, bright light exposure when it’s time to wake up, decreasing your exposure to bright lights after your shift, and wearing blue light-blocking glasses after your shift to encourage melatonin production.  

For our full guide to sleeping on the night shift, click here

The Last Word from Sleepopolis 

Light plays a crucial role in regulating our sleep-wake cycle or circadian rhythm. Daylight exposure can have a positive effect on sleep as it helps to regulate our circadian rhythm and promote wakefulness during the day, whereas exposure to light at night can disrupt our sleep and make it difficult to fall asleep or stay asleep. And while bright light in the morning is good for us in many ways, blue light can be detrimental to our sleep. It can impede melatonin production and shift your circadian rhythm, ultimately delaying sleep onset and leading to daytime sleepiness. The best sleep environment is absolutely dark, quiet, and cool.  

Sharon Brandwein

Sharon Brandwein

Sharon Brandwein is a Certified Sleep Science Coach and a freelance writer. She specializes in health and beauty, parenting, and of course, all things sleep. Sharon’s work has also appeared on ABC News, USAToday, and Forbes. When she’s not busy writing, you might find her somewhere curating a wardrobe for her puppy.