Ever wonder why evening screentime affects some people’s sleep and not others?

Lorraine Irlam, Registered Clinical Counsellor
Sep 15
3 min read

The answer might lie in something called the Theory of Photostasis.

Decorative image of a neatly made white bed with white bedding in a modern, minimalistic bedroom.

Do you ever wonder why screentime before bed seems to mess with your sleep, but not always? Or why some people can scroll through their phones at night and still fall asleep easily?

The answer might lie in something called the Theory of Photostasis, a fascinating concept about how our eyes adapt to light throughout the day.

What Is the Theory of Photostasis?

Think of your eyes like they have a "light quota" to meet each day. The Theory of

Photostasis suggests that our visual system is wired to expect a consistent amount of light daily. If your eyes get less light than they expect, they try to compensate by becoming extra sensitive.

A close-up of a blue eye reflecting a bright window.

What Happens in the Dark?

When you're in a dark space for a while (like a dim room at night), your eyes start making more of a protein called rhodopsin. This protein helps you see better in low light. But it also makes your eyes super responsive to even small amounts of light - like the glow of your phone screen.

Because of how close your eyes are to the part of the brain that releases melatonin (your sleep hormone), even dim light can interfere with that process. The more rhodopsin in your eyes, the more likely a little screen light will keep your brain awake.

So, Why Doesn’t Everyone's Sleep Get Affected by Screentime?

Here’s the catch: screen light isn't very bright - usually only about 50 lux, which is nothing compared to sunlight. But if you were in the dark before looking at your screen, your eyes are more likely to overreact.

On the other hand, if you were doing things in regular lighting (like eating dinner or tidying up), your eyes won't make as much rhodopsin. That means your brain won’t be as sensitive to the screen light, and your sleep hormone melatonin stays on track.

People walking on a wide gravel path in a forested setting with the sun peaking from behind the trees.

Why Bright Daylight During the Day Helps

Getting natural light during the day tells your body: "Hey, you've had your fill of photons." Your eyes don’t have to overcompensate later. So by nighttime, you’re less likely to lose sleep from screentime, whether it was a quick text check or a video stream.

Takeaway

Darkness boosts rhodopsin, making your eyes extra sensitive to light
Bright screen light feels stronger after darkness, potentially suppressing melatonin
Getting plenty of daylight and avoiding total darkness before screen time may help protect your sleep

Want Better Sleep?

Try these simple tips:

Spend time outdoors during the day
Keep evening lighting soft but not pitch dark
Avoid scrolling in total darkness

Your sleep is worth protecting, and understanding your body’s relationship with light can make a huge difference.

Many thanks to Dr. Michael Gradisar, Head of Sleep Science at Sleep Cycle AB / Clinical Psychologist in Australia, for taking a close look at the research and making simple sense of it for the rest of us.

References:

Rångtell, F. H., Ekstrand, E., Rapp, L., Lagermalm, A., Liethof, L., Bucaro, M. O., Lingfors, D., Búcaro, M. O., Lingfors, D., Búcaro, M. O., ... & Benedict, C. (2017). Two hours of evening reading on a self-luminous tablet vs. reading a physical book does not alter sleep after daytime bright light exposure. Sleep Medicine, 23, 111–118. https://doi.org/10.1016/j.sleep.2016.10.009

Williams, T. P., & Goldsmith, C. L. (1999). Reciprocity between light intensity and rhodopsin concentration across the rat retina. Vision Research, 39(9), 1627-1634. https://doi.org/10.1016/S0042-6989(98)00271-5

Looking for support with your sleep? Book a free 15-minute phone consultation with Lorraine - your opportunity to learn more about one-on-one sessions for anxiety and/or CBT for insomnia (CBT-I).