WHEN SETTLING into bed, some might find themselves passing over a long-neglected book and reaching instead for a phone. They would not be alone. A survey of 122,058 adults, published in 2025, found 41% used a screen every night in the hour before bedtime. Only 17% reported no pre-slumber screen-use at all. The adverse effects might not just be limited progress on “War and Peace”. A number of studies have linked evening screen-use to a subsequent bad night’s sleep.
Light
WHEN SETTLING into bed, some might find themselves passing over a long-neglected book and reaching instead for a phone. They would not be alone. A survey of 122,058 adults, published in 2025, found 41% used a screen every night in the hour before bedtime. Only 17% reported no pre-slumber screen-use at all. The adverse effects might not just be limited progress on “War and Peace”. A number of studies have linked evening screen-use to a subsequent bad night’s sleep.
Light from screens often has a lot of blue in its spectrum, leading some to suggest that blue is to blame for poor kip. Retailers have taken note. A gaggle of products to filter blue light is now available, including glasses, software and screen attachments. But the evidence justifying such bias against blue hardly passes with flying colours.
The body’s daily rhythm is run by the brain’s circadian clock. This promotes alertness in the morning and drowsiness in the evening. Light sets the tempo. A handful of cells in the eye contain a light-sensitive pigment called melanopsin. These cells do not assist vision but instead send signals to the circadian clock’s neurons. That suppresses production of melatonin—a hormone which induces night-related physiological processes—and thereby synchronises the body to Earth’s day-night cycle.
Melanopsin is particularly sensitive to light with wavelengths of around 480 nanometres, squarely in the spectrum’s blue zone. That such light blocks melatonin is thus uncontroversial. But judging the impacts of everyday exposure is another matter.
A study published in the Proceedings of the National Academy of Sciences, in 2015, asked a dozen participants to read either an electronic or a print book in an otherwise dimly lit room before hitting the sack. The authors found those reading screens had a roughly 50% reduction in melatonin compared with print readers. Screen readers also took longer to fall asleep. Case closed, you might think. But the participants were asked to read for four hours—a lot even for a bookworm—after which those with square eyes took an average of just ten minutes longer to doze off.
Further complicating matters is a similar study from 2016, in Sleep Medicine. The authors lowered the reading time for each group to two hours before bed and also exposed participants to a constant bright, white ceiling light for 6.5 hours during the day. They found no differences between groups in melatonin levels or the onset and duration of sleep.
According to Russell Foster, a circadian neuroscientist at Oxford University, blue-light boffins are swamped with such complications. He warns against extrapolating from laboratory experiments like these to the real world, citing a lack of data.
The power of blue-blocking spectacles is similarly blurry. A review published in 2023 in the Cochrane Database of Systematic Reviews analysed 17 trials of such lenses and found no clear evidence they altered sleep. The effectiveness of blue-filtering software for screens is inconclusive, too.
Dr Foster does say that it makes sense to minimise light exposure before you go to bed. But that is not a recommendation to eschew blue frequencies in particular. Bright white light, regardless of its source, increases alertness and can shift the circadian clock. “How bright and for how long? We don’t even know that,” he says. “As long as it’s not too bright, you’re probably fine.”
One Subscription.
Get 360° coverage—from daily headlines
to 100 year archives.
Archives
HT App & Website