How Light Affects Sleep


Have you ever noticed how much easier you sleep when you’re in a dark room? There’s a reason for that. You may be surprised to learn that of all the things you encounter on a daily basis, light has the biggest impact (1) on your sleep. Your circadian rhythm, or sleep-wake cycle, is attuned to the patterns of the sun: when it’s light, you feel awake, and when it’s dark, you feel tired.

However, sunlight isn’t the only source of light in your life. You are exposed to all kinds of light during the day, both natural and artificial, and they can all impact your sleep.

How Does Light Affect Your Circadian Rhythm?

Your circadian rhythm and light are very much connected. The word circadian comes from the Latin words for “about day” (2). These root words reflect the fact that your circadian rhythm follows a 24-hour internal clock that roughly aligns with the patterns of the sun. When your eyes perceive light, they send a message to your brain. Then, depending on the strength of the perceived light, your brain interprets what time of day it is and updates internal biological processes (3) accordingly, from your energy levels to your appetite.

When the only light you are exposed to is natural, this system works well. You feel awake and tired in sync with the rising and setting of the sun (4). However, in today’s always-on world, your eyes are exposed to artificial light at all hours of the day, from the fluorescent lights at your office to the beaming blue lights of your smartphone.

How Does Artificial Lighting Affect Your Sleep?

When your exposure to artificial light is extended, or if the light is particularly strong, it can disrupt your circadian rhythm — especially when the exposure occurs at night, when humans are more sensitive (5) to light.

For many people, this manifests as feeling wide awake late at night, fueled by hours of scrolling on social media or watching TV. Even short bursts of exposure to artificial light can cause difficulties falling and staying asleep. Chronic disruptions to your circadian rhythm can lead to other problems beyond sleep, such as an increased risk of obesity, mental health disorders (6), and cancer (7).

Once you fall asleep, artificial lighting can still impact how restful your sleep is. During sleep, your brain cycles through four stages of sleep several times per night, from light sleep to deep sleep to rapid eye movement (REM) sleep and back again.

Excessive exposure to light at night can disrupt the transitions (8) between these cycles of sleep. Your sleep quality suffers, and you may even find yourself waking up during the night, decreasing the amount of time you spend in essential REM sleep. The following day, thanks to your poor sleep (9), you may feel less refreshed upon waking up and feel grumpier and less focused than usual.

How Does Artificial Light Affect Melatonin Production?

Melatonin is a hormone that induces drowsiness. Your melatonin levels rise and fall in tune with your sleep-wake cycle, rising in the evening to induce sleepiness, and lowering in the morning to wake you up. Your brain relies on light cues to determine when to initiate melatonin production.

If you get a lot of light exposure in the morning, you feel sleepy earlier. The problem arises when you’re exposed to artificial light in the evening, when your brain naturally wants to produce more melatonin. Because it perceives light instead of darkness, your brain assumes it’s still daytime and inhibits melatonin production, causing you to feel awake even if it’s your bedtime.

Are There Any Sleep Disorders Associated With Circadian Disruption?

A misaligned circadian rhythm can lead to disordered sleep, both on a short- and long-term basis.

When you experience jet lag, what you’re really feeling is the effects of having a circadian rhythm that’s out of sync with your external environment (10). When you quickly travel across time zones, as happens with air travel, you suddenly find yourself in an environment where the sun’s patterns don’t match where you came from. Until your circadian rhythm adjusts to the new environment, you can feel groggy, unwell, and have trouble falling asleep or waking up at appropriate times.

Individuals who work late shifts or night shifts can also suffer from circadian misalignment since they typically have to spend time sleeping during the day and being awake at night. Shift work sleep disorder is a circadian rhythm sleep-wake disorder that affects 10% of shift workers (11). Symptoms include excessive sleepiness, lower productivity, and an increased risk of workplace accidents.

When the daylight shortens in the winter, some people who live far from the equator may develop a form of seasonal depression known as seasonal affective disorder (SAD) (12). People with SAD may struggle with sleeping too much or too little, low energy levels, and carbohydrate cravings that lead to weight gain.

Light therapy is commonly prescribed as a treatment for these disorders. This therapy is done at home with the use of a specialized light box designed to mimic sunlight. Individuals sit in front of the box each day for a set period of time, usually 30 to 60 minutes. They’ll either use the light box in the morning or at night, depending on whether they want to go to bed earlier or later.

What Is Blue Light, and How Does It Affect Your Sleep?

All wavelengths of light can affect sleep, but some are more impactful than others. For example, in one study, participants were exposed to either blue light or green light for 6.5 hours at night (13). The blue light delayed melatonin production for twice as long. Blue light has a particularly strong impact (14)  on your circadian rhythm.

LEDs and electronic screens (15) emit blue light, which has a short wavelength (16) that your brain interprets as sunlight. Like sunlight, blue light improves focus and mood. It also keeps you awake. When your brain perceives blue light, it delays melatonin production and sleep onset.

This means that if you spend your evenings flooding your eyes with blue light from your phone, computer, or TV screen, you could be unknowingly pushing back your bedtime.

What Are Some Tips for Better Sleep?

If you suspect that artificial light is messing with your sleep, there’s good news. A few small changes can make a big difference towards reducing your exposure and improving your sleep.

Make Your Bedroom as Dark as Possible

The darker your bedroom, the better your sleep. It’s as simple as that. Keeping the lights on, however dim, can disrupt your sleep, even with your eyes closed. You may wake up in the middle of the night, or earlier than you want to.

Bedroom lights can negatively impact other aspects of your well-being as well. Sleeping with lights on can lead to eye strain (17) and disrupt your metabolism (18). One study found that women who slept with the TV or a lamp on were more likely to gain weight or become obese, independent of their diet or exercise habits.

Follow these tips to darken your bedroom and improve your sleep:

  • Use blackout curtains to block out light from the sun and the street.
  • Turn off all bedroom lights when you go to bed, and use dimmer lights in the hours beforehand.
  • Turn off all electronics and keep your smartphone out of your bedroom.
  • If you keep your phone with you, lock the screen and turn off notifications to avoid it lighting up.
  • Turn clock faces away from you and cover up any other electronic lights.
  • Wear an eye mask to block out any additional light.

If you prefer to have a nightlight in your room, choose one with a dim red bulb. Red wavelengths have minimal to no impact on your circadian clock. Better yet, choose a motion-activated one so it only turns on when you need it.

Monitor Your Light Exposure Throughout the Day

Beyond your bedroom, being thoughtful of your light exposure can make it easier for you to sleep at night and feel energized during the day. Try these tips:

  • Reduce your exposure to bright lights in the evening. Dim the lights or use warmer bulbs to signal to your brain that it’s time to get ready for sleep.
  • Reduce your electronics use a few hours before bed, and stop using electronics altogether at least one hour before bed.
  • Turn on night mode on your phone and tablet in the evening.
  • Try wearing blue light blocking glasses when you’re using your phone or computer. These may filter out some blue light and relieve insomnia symptoms.

Finally, get plenty of natural sunlight throughout the day. This will give you a nice mood and energy boost, while helping realign your circadian rhythm. The research shows that for each hour you spend outside, you can advance your circadian rhythm (fall asleep earlier) by 30 minutes.


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