The Complete Guide To Light And Sleep

Written by Sharon Brandwein, Health Writer

Methodology

At Sleepopolis, we’re constantly monitoring new sleep health developments in order to keep our articles as up-to-date and accurate as possible.

Just Updated

This article has been updated to reflect the most recent scientific consensus around light and sleep.

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Long Story Short

• Light is the key factor in regulating the body’s internal clock, or circadian rhythm. (1)
• Light affects sleep in both positive and negative ways. Natural sunlight helps promote wakefulness during the day and regulate our circadian rhythm, whereas artificial light and blue light from devices can hamper melatonin production, delay sleep onset, and disrupt circadian rhythm. (2, 3)
• Exposure to daylight is shown to help you sleep longer, feel more tired in the evening, and help you fall asleep faster. (1)
• Blue light during the day is vital for daytime alertness; however, exposure to blue light at night can disrupt our sleep cycle, potentially leading to serious health issues. (4)
• Exposure to artificial light at night while sleeping may be a risk factor for weight gain and obesity, as well as breast cancer. (5, 4)

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 regulating 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. 

Note: The content on Sleepopolis is meant to be informative in nature, but it shouldn’t be taken as medical advice, and it shouldn’t take the place of medical advice and supervision from a trained professional. If you feel you may be suffering from any sleep disorder or medical condition, please see your healthcare provider immediately. 

How Does Light Affect Sleep? 

Today, we know that our internal body clock — also known as our circadian rhythm — operates 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, M.D., 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.” Moreover, different light wavelengths can have varying effects on sleep. For example, green light hastens sleep onset by as much as three minutes, whereas violet and blue light have been shown to delay sleep by as much as 10 and 19 minutes, respectively. (6)

Beyond regulating our sleep and circadian rhythm, light uses photoreceptors in the eyes to control and regulate a host of biological processes, like melatonin production, digestion, body temperature, and energy and activity levels, across a 24-hour period. (7)

Sunlight and Circadian Rhythms 

The circadian rhythm is the internal clock that regulates our body’s sleep and wake cycles over a 24-hour period. 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 is controlled by the part of your brain that responds to light — that means light, be it sunlight or artificial light, can profoundly impact its inner workings. (8)

With sleep so closely linked to light, it stands to reason that changes in daylight across seasons can impact on our sleep. (9) If you’re one of the many whose sleep schedule goes off the rails in the spring and fall due to the start and end of daylight saving time, you’ve experienced these issues yourself. 

Circadian Rhythms and Daylight Saving Time

When we spring forward in March, we might gain an hour of sunlight in the evening. However most people don’t realize that additional sunlight can delay our melatonin production. For those who don’t know, melatonin (aka “the sleep hormone”) is responsible for making us feel sleepy in the evening, so blips in its production can end up toying with our sleep and throwing off our circadian rhythm. (10) Likewise, when we fall back at the end of daylight savings time, the shift in sunset time can leave us feeling sleepy earlier in the evening, which also disrupts the circadian rhythm. 

Light and Seasonal Affective Disorder (SAD)

When changes in environmental light occur outside of the timing we’re used to (i.e., the timing of sunrise and sunset with changing seasons), it can meddle with our sleep in the form of Seasonal Affective Disorder (SAD). (11) 

We’ll note here that the term Seasonal Affective Disorder (SAD) is a bit outdated. The current terminology is “Major Depressive Disorder, recurrent episode, with seasonal pattern.” For our purposes, however, we will use SAD throughout this article, as the newer terminology is not widely used and understood as yet. (12)

Winter-pattern SAD (early sunsets) may cause an overproduction of melatonin, which leaves us feeling sleepy earlier, and the deficit of daily sunlight can lead to depression symptoms. (11) On the flip side, summer-pattern SAD may reduce melatonin levels, worsen sleep quality, and trigger depression symptoms as a result. (11)

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. (13) 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, Robert Oexman, a clinical sleep researcher at the Stanford University Sleep Research Center 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.” (14) 

While light typically halts its production, Oexman clarifies that melatonin production as it relates to light is not an all-or-nothing kind of deal. “[Melatonin production] can happen even when we’re exposed to small amounts of light,” he says, adding, “However, a small amount of light won’t completely stop your melatonin production, but it will dysregulate its production.” (15)

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.’”

Sleep Cycles 

Light can also affect sleep architecture (the basic organization of our sleep and how we move through sleep cycles), and the result could be a fierce blow to your sleep quality. (16, 1)

“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.” Existing research suggests that light exposure may cause more nighttime awakenings and less time spent in stage N3 or deep sleep. (1, 15) 

Circadian Rhythm Disorders

In the world of sleep disorders, a handful are linked to light exposure. Known as circadian rhythm disorders, these sleep-wake disorders occur when there’s a misalignment between a person’s internal clock and the external light/dark cycles around them. (17) Some common circadian rhythm disorders are jet lag, shift work disorder, and non-24-hour sleep-wake disorder (N24). (17)

Jet Lag

Jet lag is a sleep-wake disturbance that happens when your internal circadian rhythm is out of sync with the local time zone to which you traveled. Jet lag is usually a result of day-night cycles getting flipped around or shifted and light exposure that deviates from your normal schedule.

Though temporary, common symptoms of jet lag include difficulty falling asleep, daytime sleepiness, irritability, fragmented sleep, and fatigue. (18)

Shift Work Disorder

Shift work disorder is another circadian rhythm sleep disorder caused by ill-timed light exposure. As its name implies, shift work disorder typically affects people whose work patterns deviate from the standard nine to five. It’s most common among healthcare workers, law enforcement, and military personnel who are required to work night shifts. As a result, they must try to sleep during the day when circadian alertness is at its highest. As a result, shift workers tend to experience short sleep and poor sleep quality. (19)

Non-24-Hour Sleep-Wake Rhythm Disorder

Non-24-hour sleep-wake disorder (N24) is a circadian rhythm disorder in which the body’s internal clock fails to sync to a 24-hour day. (17) Typically, those with N24 have delayed sleep times ranging anywhere from a few minutes to hours. With each passing day, sleep start times for those with N24 become later and later, eventually making their way around the clock. (17)

Because the retina cannot transmit natural light to the hypothalamus portion of the brain, N24 is rare among the sighted population and more common in people who are blind. (20)

What Types Of Light Affect Sleep? 

Light can affect our sleep in both positive and negative ways. Timing matters — natural light (sunlight) 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 (one of the deeper sleep stages, which are the most restorative). (1)

Blue Light

Whereas daylight positively affects your sleep, blue light can be quite unhelpful. 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 and can shift circadian rhythms by as much as three hours, delivering a powerful one-two punch to your sleep. (21) 

If you’ve paid a bit 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.” 

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 should take care not to “double down on other behaviors that can affect your sleep.” 

Benefits of Sleeping in a Dark Room  

Your circadian clock is extremely sensitive to light, so a dark room (or sleep environment) is crucial to quality sleep. “A sleep-friendly environment is one that is dark, cool, and quiet,” says Joseph Dzierzewski, Ph.D., vice president of research and scientific affairs for the National Sleep Foundation, 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. (22)

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

Beyond impairing your sleep and throwing your circadian rhythm out of whack, some evidence suggests that sleeping with the lights on is bad for your health in more ways than one. 

While one small 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, and another found that artificial light at night can be a risk factor for weight gain and obesity. (22, 23)

How To Optimize Your Light Exposure For Better Sleep 

Sleep hygiene guidelines state that sleep environments should be cool, quiet, and dark. For some, however, managing light can be a challenge from time to time. To optimize your sleep space, you might try any of the following. 

Get Natural Light in the Morning 

Exposure to sunlight in the morning suppresses the secretion of melatonin in your brain to help you shake off the sleep and feel more alert. (1) It can also help regulate your circadian rhythm, leaving you appropriately tired at the end of the day and more apt to fall asleep with ease. (24)

Dim the Lights in Your House at Night 

While sunlight throttles your body’s melatonin production, leaving you feeling more awake and alert, exposure to light at night can have the same effect. Obviously, this is not what you want at the end of the day, so to keep things working as they should, you might try dimming the lights around your home in the evening hours. (25)

Wear a Sleep Mask to Bed 

While most of us may think that shutting off the overhead lights in our bedroom or turning off our bedside lamps may be enough to leave our bedrooms dark enough for sleep, light can be pretty sneaky. Whether you’re talking about slivers of light coming through the window blinds or the light from devices and bedside clocks, all of the above can meddle with your sleep. In this case, sleep masks are an excellent way to manage these small light sources.

Get Blackout Curtains 

If windows and streetlights are problematic, you might consider blackout curtains to make your sleep space pitch black. 

Put Away Electronics at Night 

A 2024 Sleepopolis study found that almost 90 percent of participants regularly indulged in screen time before bed. (26) The problem here is that by hampering our melatonin production and triggering emotional responses, device use before bed can do more harm than good. (27) To protect your sleep, consider turning off your electronic devices at least one hour before bed to give your body and mind some time to relax and prepare for sleep. (28)

Change light settings on electronic devices 

Another option for managing your exposure to blue light at night is using the night mode setting, which seems to be standard on most smartphones. 

FAQs

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.  

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Dzierzewski, Joseph. Author Interview. August 2023.

Oexman, Robert. Author Interview. August 2023. 

Wells, Audrey. Author Interview. August 2023.