ONE SPECTRUM.
DIFFERENT EFFECTS.
Your screen emits visible light. Your phone also uses radio signals. Understanding the difference helps us ask better questions about both.
Your screen and your signal do different jobs.
The screen sends light toward your eyes. The phone’s radios send and receive signals to stay connected.
Turning down screen brightness changes the light. Switching off Wi-Fi changes one radio connection. Those are different settings, so they call for different evidence.
“EMF” covers
a lot of territory.
Frequency, intensity and the way energy interacts with the body matter. A finding about one part of the spectrum cannot simply be applied to another.
Conceptual diagram; bands are not to scale. Shown here: non-ionizing regions. Spectrum background · National Cancer Institute ↗
From light to a timing signal.
Light reaches the eye.
Timing, spectrum, brightness and duration influence the signal. Screen colour alone does not describe the exposure.
The eye passes the signal on.
Light-sensitive cells in the retina—the tissue at the back of the eye—help with vision and send timing information to the brain.
The brain gets a time-of-day cue.
In controlled studies, evening light changed melatonin (a hormone involved in nighttime signaling), body-clock timing and alertness.
Pathway and exposure context: Brown et al., 2022 ↗. Early human action-spectrum experiment: Brainard et al., 2001 ↗.
What the screen
study actually tested.
The same 12 people tried both: reading on a light-emitting device and reading a printed book. Researchers changed the order between participants and compared the results.
Adult participants.
Each person experienced both reading conditions in a controlled inpatient study.
Before bedtime.
Participants read for approximately four hours on five consecutive evenings in each condition.
The body clock ran later.
The device condition also reduced melatonin and next-morning alertness compared with print.
Evening light-emitting reading conditions can affect sleep-related physiology under the tested conditions.
How much the results apply to shorter sessions, different brightness settings or ordinary home use. The experiment tested reading conditions, not this website’s dark theme.
The time of day belongs
in the picture.
Expert consensus for healthy adults describes a daily light–dark pattern. The recommendations consider light reaching the eye, individual differences and enough light to see safely.
Daytime
A stronger daytime light signal helps distinguish day from night.
Evening
Lower light exposure in the hours before sleep reduces the evening circadian signal.
During sleep
A dark sleep environment supports the intended light–dark pattern.
Based on Brown et al., 2022 · PLOS Biology ↗. Expert guidance is not a trial showing disease prevention. Sleep and circadian findings do not establish that screen light causes cancer.
Why does this site start dark?
It is a display preference. Choose whichever theme is easier for you to read. Neither version is presented as a medical intervention.
Beyond the brightness
outside your window.
Adults in UK Biobank.
Participants wore light sensors for one week.
Years of follow-up.
Researchers compared those light patterns with deaths recorded during follow-up.
An association.
Brighter nights tracked higher mortality; brighter days tracked lower mortality.
Personal sensors capture more of someone’s environment than satellite estimates of outdoor light.
A sensor on the wrist does not measure exactly how much light reaches the eyes. One week cannot represent a lifetime, and other factors could contribute. This does not prove dark mode prevents disease.
Windred et al., 2024 · PNAS ↗ · Read the source and add it to a response →