Do screens and blue light before bed ruin your sleep?

That evening light affects the circadian rhythm is well established. Short-wavelength blue light (around 460–480 nm) hits the melanopsin in the retina's ganglion cells, which signal the central clock to slow melatonin down. In a controlled study by Chang and colleagues, participants read either a light-emitting eReader or a printed book before bed. The eReader suppressed melatonin, made people fall asleep more slowly, pushed the internal clock later, reduced REM sleep and left them less alert the next morning. So the mechanism is real.

The question is how much it matters for how you actually sleep. Cajochen and colleagues pooled studies where sleep was measured objectively with polysomnography. For a single evening exposure they found no clear effect on sleep onset, sleep efficiency or deep sleep; all three estimates were statistically uncertain. The effect on sleep itself is, in other words, modest, not the catastrophe the word "ruin" implies.

And the most common fix does not deliver what it promises. A Cochrane review by Singh and colleagues examined blue-light filtering glasses and found six randomised trials pointing in different directions, at very low certainty. We simply do not know whether the glasses improve sleep. What clearly disrupts sleep is a late hour, bright light and content that keeps the brain running.

One striking lab study got wide media coverage, and the takeaway "screens ruin your sleep" is easy to remember and share. It also fits a comfortable story: the fault sits in an invisible wavelength, not in scrolling until midnight. That turns the problem into something you can buy your way out of. Blue-light glasses, screen filters and "night mode" are sold and marketed as sleep savers, and a real but small melatonin effect gets inflated into a promise they barely keep. A tangible gadget feels like a solution, while the duller advice, put the phone down earlier, cannot be sold.

If you want to sleep better, aim at what actually matters most: dim the lights and lower your screen brightness for the last hour, avoid stimulating or stressful content, and keep a steady bedtime. Night mode and blue-light glasses cost little and do no harm, so use them if you like, but don't count on them to rescue a night. If the phone is the last thing you do before sleep, the hour and the alertness it creates are a bigger problem than the colour of the light.