There are many chapters and verses in the gospel of good sleep, from chamomile tea to a firm mattress and the ideal bedroom temperature. Arguably, though, the golden rule of our modern preoccupation with good sleep is a simple one: no blue light before bed.
This is a simple commandment but one which speaks to our times. After all, what are the biggest emitters of blue light? The phones, tablets, laptops, TVs and games consoles that we busy ourselves with in the hours leading up to bedtime.
The 20-year-old theory revolves around the discovery of a type of cell in the eye called intrinsically photosensitive retinal ganglion cells (ipRGCs for short.)
“These cells don't respond to visible light directly so they're not part of the visual receptor,” explains independent sleep expert Dr Neil Stanley. “They respond specifically to blue light in the range of about 460-480 nanometres. Or so it was thought.
“Because they respond specifically to blue light and they have no participation in image forming, it was believed they played a part in the circadian control of sleep. In other words, blue light was thought to control when we fall asleep and when we wake up. The mechanism for this was assumed to be via melatonin [a hormone which causes feelings of sleepiness].”
In short: blue light suppresses the 'sleep hormone' melatonin and avoiding screens for two hours before bed will help release melatonin, setting off a chain of events which result in a good night's sleep.
However, it might not be as straightforward as that. A recent study on mice from the University of Manchester found that comparing exposure to different colours of light, it was actually yellow light which disturbed sleep more than blue. This led the researchers to hypothesise that warmer light colours might make the body think that it’s daytime, while cooler blue light is more evocative of twilight causing the body to release more of the sleep hormone melatonin.
So does this effectively rewrite the blue light theory as we’ve all come to understand it? Well, maybe, but this is far from the first time that 20-year-old nugget of wisdom has been challenged by modern science, says Stanley.
“The problem is that the theory is vastly more complex. We now know from research, such as the study from Manchester, and another study from around the same time shows there are three distinct types of these ipRGCs.
“And we also know that you need a dose of blue light to be awake. If you spend all day and all night working outside under a bright blue sky, using your phone or tablet at night seems to make no actual difference. Whereas if you sit without any exposure to blue daylight, that's when the screen usage will have an effect.”
And therein lies the trouble: a lot of the science around blue light has dealt with laboratory conditions rather than the way humans actually come into contact with blue light. And even then, the effects are not quite so extreme as we’ve been led to believe.
For example, one widely-cited 2014 study from the University of Texas found that reading on an iPad before bed suppresses melatonin production for an hour and a half, compared to readers of paper books. However, the same study found the suppression of melatonin delayed the onset of sleep by just ten minutes.
“It's not simply that blue light at night is bad because we know that [devices which block blue light] also disturb sleep,” continues Stanley. “We know that stripping out blue light using Night Shift mode or whatever on your phone makes no difference because it makes your screen muddied, so you turn the brightness up, and brightness is as bad as blue light in that regard.”
As far as Stanley is concerned, blue light is only part of the problem, forming a triumvirate of issues along with screen brightness and the fact that playing games, catching up with friends, or working on a phone or tablet is inherently stimulating.
The other complication in the blue light orthodoxy is that light affects all of us differently. “There was some research from Australia which shows that people's sensitivity to light varies hugely,” says Stanley. “For some people, even just the light from a candle will be sufficient to disturb their sleep whereas others will be able to look at quite bright light from a 60 watt lightbulb without any impact.”
As for blue light-blocking devices like filters and special glasses? “The claims that people make for these things are vastly in advance of where the science actually is,” says Stanley. “I think it's just playing the gullibility of people: you set up that there's a problem and then you come up with and sell a device to fix it.”
While there certainly appears to be a link between light and sleep, it’s worth remembering that the science is still far from settled on the matter.