How Light Affects Your Sleep
Light plays a major role in how your body knows when to wake up, when to feel alert, and when to prepare for sleep. This process is largely guided by the suprachiasmatic nucleus, or SCN, which acts as the body’s master clock.
The light-detecting cells in our eyes notify the SCN when there is light outside. These cells are especially good at detecting blue light, which is naturally found in sunlight. This is helpful in the morning because it tells the body that the day has begun. The problem is that blue light in the evening can send the body the wrong message.
When the body is preparing for sleep, exposure to blue light can make it harder for the brain to recognize that night has arrived. In practical terms, this means the timing, amount, and source of light we are exposed to can influence how well we sleep.
Sleep experts commonly point to four important strategies.
Get Sunlight First Thing in the Morning
Being exposed to sunlight first thing in the morning sends your SCN a simple message: good morning.
After spending the previous few hours in darkness, your system is more sensitive to light in the morning. That means even a relatively small amount of morning light can be effective in helping your body recognize that the day has started.
This does not mean looking directly at the sun. You should never stare at the sun because it can permanently damage your retina. The goal is simply to get natural outdoor light into your eyes safely.
Spend Time Outside During the Day
Spending time outside during the daytime helps as well, even when it is cloudy. Outdoor light is still much brighter than indoor light, which is one reason daylight exposure can be so useful for supporting the body’s internal rhythm.¹
Being outside helps make the master clock more robust. It also helps synchronize that central clock with the outside day and with the peripheral clocks throughout the body.
In other words, light exposure is not only about waking up in the morning. The light you get throughout the day helps reinforce your body’s internal rhythm.
Be Mindful of Screens at Night
Blue light-emitting screens can interfere with sleep, especially when they are used close to bedtime. Some experts recommend avoiding screens at least an hour before your usual bedtime.¹
The effect may depend on the type of screen and how close it is to your face. A television across the room does not appear to be as disruptive as a phone, tablet, or computer screen held close to the eyes. The amount of natural light you get during the day may also matter. If you were exposed to a lot of outdoor light earlier in the day, you may be less affected by screen light at night.¹
Children are more sensitive to light, which means they may be more affected by evening screen exposure. A 2024 National Sleep Foundation consensus statement found that screen use can impact sleep health across the lifespan, with special concern for children and adolescents.²
Sleep in a Cool, Dark Room
Many experts agree that a good sleep environment should be cool and completely dark. If you wake up in the middle of the night, it is best to avoid turning on bright lights, especially devices that emit blue light, such as a phone or tablet.
The evidence for this recommendation is strongest in children, though the body of evidence continues to evolve for other age groups. As sleep researcher Erin Flynn-Evans explained, “The influence of light never ceases to amaze me in that every year it seems we learn something new about how powerful light is and how [even] little light exposure is impactful.”¹
The Bottom Line
Your body is constantly paying attention to light. Morning sunlight helps tell your brain the day has started. Daytime outdoor light helps strengthen your internal clock. Evening blue light can confuse that system, especially when it comes from screens close to your face. A cool, dark room helps protect the sleep environment your body needs.
Better sleep does not always begin at night. Often, it begins with the light you get first thing in the morning and the light you choose to limit before bed.
References
“Screen Time and Sleep: It’s Different for Adults,” Restorative Sleep, Stanford Lifestyle Medicine, August 8, 2024. https://longevity.stanford.edu/lifestyle/2024/08/08/screen-time-and-sleep-its-different-for-adults/
Lauren E. Hartstein et al., “The Impact of Screen Use on Sleep Health across the Lifespan: A National Sleep Foundation Consensus Statement,” Sleep Health 10, no. 4, August 2024, 373–384. https://doi.org/10.1016/j.sleh.2024.05.001