Melatonin, a hormone that regulates the sleep-wake cycle in mammals, is regulated by exposure to light. Decreasing the amount of light in our environment – especially the blue light from the various screens that surround us – is crucial for a good night's sleep. In fact, research shows that reading a book on a device rather than reading an actual book reduces melatonin production by as much as 20 percent. In this clip, Dr. Matthew Walker describes the negative effects of blue light exposure on melatonin production and, subsequently, sleep.

Matt: The first is darkness. You really do need some degree of darkness at night to release that hormone melatonin, which helps trigger the timing of the healthy onset of sleep.

Rhonda: And you would say about maybe four hours before your bedtime, or what would you say...?

Matt: Yeah, I would say three to four hours is the time to start thinking about your light saturation exposure, certainly in the last hour before bed. And there are ways that you can do that you can install software on your computer if you really have to look at it, but I would advise against that because I think computers cause and trigger anxiety. I think, in fact, if anything, my estimation right now is that the blue light from those screens is detrimental. I think the evidence is favoring a blockade of melatonin and a reduction in REM sleep.

They did this great study. They took people with an iPad, one hour of iPad reading versus one hour of book reading. iPad reading dropped or blunted melatonin by over 20%. The peak of melatonin didn't arrive until three hours later. This is one hour of iPad reading. They had less REM sleep, and they...

Rhonda: They were reading, like...?

Matt: They were reading just a book under dim light versus reading the same book on an iPad. When they woke up the next morning, the people who read the iPad felt more unrefreshed. They subjectively knew that they had not slept as well. What was interesting is that when they stopped the iPad reading, there was a washout effect that was a blast radius of reading the iPad. And it's continued into subsequent nights even though they'd stopped reading the iPad.

So I think light is a feature, but I also think part of the problem with computers and iPads and iPhones...and I don't mean to sound like a prude about this, but they do trigger anxiety. They are what I would describe as anxiogenic pieces of hardware. Unless you are, and if you can do this, please write to me and tell me how you do this, but most people get a lot of their anxiety infusion in part through these devices.

A recent study in teenagers are actually demonstrated that part of the reason that they were having sleep disruption by using phones was not necessarily about the light but was because of FOMO, fear of missing out, that if you weren't online, you would miss out on some key social communication. And they were suffering because of that, too.