Phones, laptops and televisions are now a fixture of the hour before bed for most Australians, and so is the advice to put them away. But the reasoning behind that advice is often reduced to a single word — “blue light” — without much explanation of what that light is actually doing inside the body. The real story involves a specific type of light-sensing cell, a very old internal clock, and a hormone that takes its timing cues from the sun.
The eye has a second job besides seeing
Vision depends on rods and cones, the photoreceptors that turn light into the images we consciously perceive. But the retina also contains a third, less familiar type of cell: intrinsically photosensitive retinal ganglion cells, or ipRGCs. These cells do not contribute much to what you see. Instead, they exist to measure ambient light and report it directly to the suprachiasmatic nucleus, the master circadian clock sitting deep in the hypothalamus.
ipRGCs are most sensitive to short-wavelength light in the blue part of the spectrum, roughly 460–480 nanometres — the wavelength that dominates daylight, and the wavelength that LED screens and modern white lighting emit in disproportionately large amounts compared with the warmer, redder light of a candle, a fire, or an incandescent bulb. This is the biological basis for calling it “blue light”: it is simply the wavelength the body’s light-detection system was built to track.
How light talks to the clock
The suprachiasmatic nucleus does not run on a 24-hour schedule by accident. Left with no external cues at all, the human internal clock tends to drift to a period slightly longer than 24 hours. What keeps it locked to the actual day is a process called entrainment, and light is the strongest entraining signal the body has — stronger than meal timing, exercise or temperature.
When ipRGCs detect bright light, they signal the suprachiasmatic nucleus, which in turn suppresses the pineal gland’s production of melatonin, the hormone most associated with sleep onset. Melatonin normally begins rising a couple of hours before habitual bedtime, a process sometimes called dim-light melatonin onset. Exposure to bright, blue-enriched light during that window can blunt or delay the rise, signalling to the brain that it is still daytime even as the clock ticks toward midnight.
Laboratory studies using controlled light exposure have shown that a few hours of screen-level brightness in the evening can measurably suppress melatonin and shift its onset later, and that the effect is stronger with shorter, bluer wavelengths than with warmer, longer ones at the same brightness. The size of the effect varies a great deal between individuals and depends heavily on total light intensity, duration of exposure and how close the light source is to the eyes — a handheld phone sits far closer to the retina than a ceiling light or television across the room.
Brightness matters as much as colour
It is easy to fixate on the word “blue” and miss the bigger variable, which is overall light intensity. ipRGCs respond to total photon load, not colour alone, so a very bright screen at any colour temperature can still suppress melatonin, while a dim, warm light close to bedtime has a much smaller effect. This is why simply switching a phone to a “night mode” colour filter, without also reducing brightness and duration of use, tends to produce only a modest change in sleep-related outcomes in research settings.
Timing compounds the effect. The same light exposure has a far larger impact on the clock when it lands in the two to three hours before habitual sleep time — the window when the circadian system is most sensitive to being pushed later — than it does earlier in the evening.
Why this matters beyond falling asleep
Circadian timing does not just govern sleep onset. It coordinates the daily rhythm of core body temperature, cortisol, digestive activity and cellular repair processes across nearly every organ system. A circadian signal that is chronically delayed by late, bright light exposure can leave the sleep-promoting and wake-promoting systems out of step with the actual clock on the wall, contributing to the experience of feeling tired at the wrong times, or wide awake well past the point of wanting to be.
This is one reason sleep researchers talk about “light hygiene” as a companion to more familiar sleep hygiene advice: consistent exposure to bright light in the morning to anchor the clock, and a gradual dimming of light in the hours before bed to let the evening melatonin rise proceed unimpeded.
Practical, evidence-informed adjustments
- Dim the environment, not just the screen. Overhead lighting in the evening often contributes more total light exposure than a phone held at arm’s length.
- Get bright light early. Morning daylight exposure helps anchor the circadian clock and can make the system more resilient to evening light later on.
- Treat night mode as a partial measure. Reducing screen brightness and shortening use in the pre-bed window tends to matter more than colour filters alone.
- Distance and duration count. A screen held close to the face for an extended period delivers more light to the retina than the same screen viewed briefly from across a room.
Frequently asked questions
Does blue light exposure affect everyone the same way?
No. Sensitivity to evening light varies with age, individual circadian chronotype, and baseline light exposure during the day. Some people notice a clear effect on how quickly they fall asleep; others notice very little.
Do blue-light-blocking glasses work?
Evidence is mixed. Some small studies suggest a modest benefit for melatonin timing when glasses block a significant portion of short-wavelength light, but results are inconsistent and effect sizes are generally smaller than those achieved by simply dimming overall light and reducing screen time before bed.
Is all screen use before bed equally disruptive?
Not necessarily. Passive, dim, low-engagement use differs from bright, prolonged, highly stimulating use. Total light dose and mental arousal both appear to play a role, which is why the effect is not identical across every type of screen activity.
Understanding how light and the circadian clock interact does not require giving up screens altogether — it means being deliberate about when and how brightly they are used. If sleep difficulties are persistent despite these kinds of adjustments, a conversation with a health professional can help identify what else might be contributing.
