Some screen use before bedtime might be OK for your sleep after all

The blue light that smartphone and laptop screens give off may not affect your sleep if you only get a small dose of it

By Jason Arunn Murugesu

The blue light from screens can delay you nodding off if you get too much of it

Realstock/Shutterstock

A bit of screen time before bed might not actually damage the quality of your sleep too much.

Several studies have found that pre-sleep exposure to blue light, which is produced by laptop and smartphone screens, can make people less sleepy and affect the quality of their rest. One of the supposed mechanisms for this is that the blue light makes bodily systems block the hormone melatonin that usually makes you feel drowsy.

To dig a little deeper, Christine Blume at the University of Basel in Switzerland and her colleagues wanted to test whether blue light that affects only intrinsically photosensitive retinal ganglion cells (ipRGCs) in the eyes would have any effect on subsequent sleep quality.

These eye cells, alongside cones and rods, are activated by light, but ipRGCs are particularly sensitive to blue light and are thought to play a major role in setting the body’s internal circadian rhythms, says Blume.

The researchers tested 29 people in a sleep lab with exposure to two types of light. The participants, with an average age of 23, all had healthy sleep histories. On one of the nights they spent in the lab, the participants were exposed to one type of light from a screen for an hour, ending 50 minutes before they typically went to sleep. The participants’ average bedtime was 11pm. About a week later, the participants had a night when they were exposed to a different light condition.

The two different lights would have looked nearly identical to the participants. However, one was made up of a high proportion of blue light, which would be picked up by the specialised retinal ganglion cells, whereas the other had a far lower proportion of blue light and so wouldn’t be picked up by these cells. An electroencephalogram (EEG) machine was used to measure brain activity while the participants slumbered.

To assess how melatonin levels changed, the researchers took saliva samples from the participants every 30 minutes in the 5 hours before the volunteers fell asleep and also took samples in the morning. The participants were questioned about how sleepy they felt before bedtime. In the morning, they were asked how well they slept and how alert they felt.

The blue light that was supposed to affect melatonin reduced levels of the hormone in the blood by about 14 per cent on average, compared with the other frequency of light, but no effect was found on self-reported sleep quality. “Melatonin and sleep are probably not as closely linked as people think,” says Blume.

She says there are several reasons why blue light may not have affected the participants’ sleep.

The need to sleep at a certain time is largely based on two components: the pressure to go to sleep, which builds during the day, and the circadian clock, which is the body’s internal clock that regulates when we need to sleep and wake up on a 24-hour cycle. The interplay of these factors also has an effect.

In young people with no particular health problems, like those participating in the study, sleep pressure may simply overpower the effects of blue light on the circadian clock, says Blume.

The study also suggests that blue light’s effect on sleep quality may be driven by other eye cells rather than ipRGCs, she says. Blume adds that had they kept the screens turned on closer to bedtime, it may have taken longer for the participants to fall asleep, but the researchers wanted to question them and let them brush their teeth.

“This study shows that exposure to bright light in the evening for a limited amount of time does not necessarily impact sleep,” says Blume. “I don’t think this study changes our overall perspective on blue light’s impact on sleep, it just adds a piece of information to the existing evidence.”

“This does not show that blue light before bed will not affect sleep,” says Stuart Peirson at the University of Oxford. “It just shows that the type of blue light they used, at the intensity the used, did not in this study.”

“I think what this paper speaks to is how complex the processes of sleep and wakefulness can be,” says Hugh Selsick at University College London. “The role of melatonin in sleep-wake regulation is well established, but it is only one of several factors that are involved in the process, such as the homeostatic sleep drive, mental state, physical health, environment, etc.”

Journal reference: Sleep , DOI: 10.1093/sleep/zsac199

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