Their brightness is “extremely unlikely” to affect the body’s sleep clock, and instead it is how long people spend on the devices, which act as a stimulant, that could make the difference.
Professor Russell Foster, head of the Nuffield Laboratory of Ophthalmology, and director of the Sleep and Circadian Neuroscience Institute at the University of Oxford, explained that the screen light simply is not strong enough.
He added that dark modes and screen filters are “misleading”, leaving users to believe it is OK to use the devices late into the night.
The Royal Society Fellow was speaking after presenting at the European Society for Paediatric Endocrinology meeting in Vienna, Austria.
He told the PA news agency that based on the data available and a study from Harvard University, you need a lot of light for a long duration to disrupt the circadian rhythm. This is the 24-hour internal clock that cycles between sleepiness and alertness at regular intervals, and is tuned to light and dark.
Professor Foster told PA: “The amount of light that you get from screens is relatively low.
“If you’re staying up all night and on the brightest screen, it might do something.
“But for most people it is not going to have a big effect.
“I think you should disentangle what is going on – it is the effect of having the phone by your bed.
“I am not saying they shouldn’t be used. I am saying that by simply saying: ‘It’s the light and not the use’, it’s misleading, because kids are then saying: ‘Oh I have got one of these devices which minimises the blue light, which means I am not going to shift my clock’.
“Yes, but you’re still using it at two o’clock in the morning.”
He added: “Dusk light makes you go to bed later, and morning light advances the clock – it makes you want to get up earlier.
“So the argument has been that evening light via these devices delays the clock – makes you want to get up later.
“Whereas what is happening is, it is probably having very little effect on the clock, it is actually just giving you much reduced sleep duration, so you’re waking up tired.”
Prof Foster said it was important to disentangle the two things.
On the basis of published data showing the effect of light on the human clock, he suggests bright light is needed to disrupt the circadian rhythm, and devices do not produce bright enough light.
“It seems extremely unlikely that it is the light effects versus the alerting effects of these devices,” he explained.
Asked about dark modes and screen filters, Prof Foster said the light mode “probably was not” bright enough to do anything in the first place.
However, he said more research was needed on their efficacy.
Prof Foster says that while it is probably sensible to minimise light exposure prior to bedtime to reduce levels of alertness, and to prepare psychologically for bed, the impact of light from digital devices needs further investigation.