Modern life is all about innovating our way out of our inabilities. We’re not meant to fly, or live 20 stories in the air, or speak to a friend in New Zealand. And (buzzkill alert) we’re not meant to be awake when the sun don’t shine. Of all that we can now do, the latter may be the most universal. If we’ve mastered anything, it’s light. For the large part, this is good news; extra hours in the “day” mean we can socialize longer and get more work done.
Trouble is, evolution has programmed the human body to equate light with wakefulness. Our eyes treat blue-heavy light — like the soft glow of an iPad — like the midday sun. If indoor lighting (hell, even fire), kept us awake well past nature’s bedtime, the perpetual glow of our devices is just another nail in the coffin of the fabled eight-hours-a-night. The problem is widespread: By some estimates, some 95 percent of Americans use a device within one hour of bed.
So until we evolve beyond a light-regulated circadian sleep/wake rhythm — or until some dystopian future renders daylight hazardous to our health — we need to accept the relationship between light and sleep, and understand what we can do to help it along. Here’s how to do just that.
Why We’re Like This
Our bodies’ responsiveness to blue light stems more from Earth’s composition than from to the color of light itself. Blue wavelengths are able to penetrate the ocean surface better than others, which is what allowed them to reach life where it first began. As species wandered onto land, the reflectance of the sky perpetuated life’s dependence on blue light for cycle-setting. (Maybe if humans had evolved on Mars, we’d have developed a similar response to red light?)
That evolution has created a complex conduit that translates light into cues that set our daily circadian rhythm (see below) A small portion of our eye’s photoreceptor cells contain a protein called melanopsin, which springs into action in response to intense, blue light. When triggered, melanopsin transmits signals to the suprachiasmatic nucleus region of the hypothalamus — aka the body’s master clock — located behind the root of the nose. In turn, signals relay to the pineal gland and suppress production of the sleep-inducing hormone melatonin.
But, despite these adaptations, the human body remains slightly out of sync with Earth’s 24-hour cycle. If left unmediated by sunlight — as several studies have tested — the human clock will run on a 24.2-hour cycle, on average. That means, if you’re locked in a windowless room and left to sleep and wake at your own whims, your clock will slip 15-30 minutes for each day of isolation; within a month’s time, day and night will be reversed.
Daylight is the key to keeping rhythms in check. “Every day when you get up, you are resetting the biological clock,” explains Mariana G. Figueiro, program director at Rensselaer Polytechnic Institute’s Lighting Research Center. “You're advancing the timing of your biological clock so that it runs with 24 hours, rather than 24.2 hours.”
That’s why morning light, which is richest in high-intensity blue wavelengths, is most important to maintaining our circadian rhythm. During sleep, our melatonin levels peak as our core body temperature bottoms out. At this point, ideally sometime around 5am, sunrise provides a melatonin-suppressing dose of blue light that eases the body towards wakefulness and syncs it with the local time. That’s why, if we wait until noon to throw open the shades, getting on-schedule becomes more difficult. “If we give light at the wrong time, it's bad,” says Figueiro, “[But] if we don't get enough light during the day, we're also going to be cranky.“
Once evening rolls around, blue wavelengths scatter and become more diffuse, giving way to the yellows, reds and pink we associate with sunset. This warmer light won’t trigger melanopsin, which allows our bodies to produce melatonin and ease into sleep.
A Struggle for Control
Of course, as scientists identify the mechanisms that regulate how the relationship between light and sleep should work, natural, geographic, and technological factors conspire to make it not work (see below)
As we age, for instance, the way our eyes deal with light changes. In a series of studies, the most recent of which published last year, Figueiro found that adolescents aged 15–17 are more sensitive to blue light (and therefore suppressed melatonin production) than other age groups; when exposed to light from devices like phones and tablets in the two hours leading up to bedtime, their melatonin levels dropped by 38 percent. This heightened sensitivity is all-but erased by the time we reach our 20s. Later in life, changes to the lens, such as yellowing or clouding, can block blue-light absorption, making it more difficult to wake and stay alert.
Our location on the globe can also play a key role. According to Anne-Marie Chang, who studies how our environment impacts sleep at Penn State University, people who live close to the poles, where daylight is either extended or shortened depending on the time of year, will eventually adapt and become able to maintain an eight-hour sleep cycle. People visiting those areas or traveling across time zones, however will have a tougher time. Jet lag sufferers know the cranky, groggy, out-of-it feeling all too well.
“You can adapt to effects of light depending on your environment, depending on the history of light exposure that you've had previously,” says Chang.” With jet lag, for instance, research suggests that you can reset to a two-hour time difference in four days or less — assuming a accumulation rate of 30-60 minutes with each passing day.
Such seasonal and geographic disruptions make electric lights something of a Catch-22. While they allow, say, Scandinavians, to work full days in the darkness of winter, they too make it far easier to outright ignore the body’s natural cycle. Artificial light allows us to work, play, or otherwise fiddle around long after our bodies want to start producing melatonin and get ready for bed. Sleeplessness and late-night partying, once blamable only on full moons, can now be a daily occurrence. “Maybe lighting is actually an enabler, because what's it's doing is it's allowing you to disrupt your sleep?” wonders Figueiro.
It’s a fair point. But living in the dark after sundown is something even cavemen wouldn’t stand for (they did, after all, discover fire), so it’s left to us to be more-mindful of what light we’re around and when. Naturally, there’s no shortage of tech solutions that color-shift to sync light with our circadian rhythm (see below), but we needn’t replace, calibrate and schedule every bulb in our homes to survive.
During the day, that means paying attention to not only the color, but the directionality and intensity of light. Photoreceptors containing melanopsin (remember that handy protein?) are clustered toward the bottom of the retina, making them most receptive to light that shines down into the eyes. Sunlight (duh) has an easy time reaching them; similarly, artificial light in offices, schools, and hospitals is best when it comes from above. At the same time, the melanopic response requires bright light as much as it does blue light.
At night, the opposites are true. That means dimming lights and killing overheads in favor of table lamps, both of which work to prevent as much light as possible from reaching melanopsin-containing receptors. Bad news: Illuminated e-readers, phones and tablets, which shine light right into your eyes are a no-no, but old-fashioned paper books are okay.
“If you're reading a book with a lamp, that light source is directed at a page, or it's directed away from you; you're not looking directly at it,” says Chang.
More bad news: Activating Night-Shift mode or other color-changing app is not a get-out-of-jail-free card; changing the color without reducing brightness will still stop melatonin production.
Now a bit of good news: According to Figueiro, pitch black is not a necessity. A bit of streetlight or a nightlight in the hallway shouldn’t throw you off. “The circadian system is blind to that light. You're better off having a 7-watt, warm-color nightlight than trying to navigate the space in the dark,” she says.
At the end of the day, experts agree that we need to accept our bodies’ response to light. It’s not something we can ignore or innovate our way around. In fact, the more we try and hack, skirt or otherwise mess with our natural pattern, the more we set ourselves up for a perpetual case of the Mondays.