How Cities Mess With Our Sleep Cycles
Electrical lighting is arguably one of the best things that's ever happened to cities. It's made this possible. And this. And this. It's unshackled the work day – and human productivity – from daylight. It's opened up a whole realm of nighttime activities: going to the theater, dining out, driving after sunset. Simply illuminating streets has made them safer, to say nothing of what the little light bulb has done for the interiors of our private homes.
All that said, the age of electrical lighting has also changed us in a pretty fundamental way.
"Scientists for many, many years now have talked about how electrical lighting in our built environment, especially in the evening and nighttime hours, has pushed the timing of our internal clocks later," says Kenneth Wright, the director of the Sleep and Chronobiology Laboratory at the University of Colorado at Boulder.
But to prove exactly what that means, no one had ever conducted an experiment before quite like one Wright recently directed, described in the journal Current Biology. He and several colleagues studied the circadian rhythms of eight people going about their heavily illuminated daily lives. The subjects wore research watches that included both an accelerometer (to measure movement and, by association, sleep) and a light sensor (to measure their exposure). The subjects did this for a week, then spent a rather intensive day in the laboratory giving saliva samples every hour for 24 hours, to measure the hormone melatonin that reveals the internal clock in our bodies.
Then came the fun part: All eight subjects went camping in the Rocky Mountains, far from city light, without so much as a flashlight to read by. After a week of living only by the sun's schedule (again, wearing these watches), they returned to the lab.
It turned out that these people were exposed in nature to four times' as much light as they normally experienced in the electrically illuminated world. And their varying internal clocks – some of them had been night owls, some early birds – all adjusted in sync with the sun, within the span of only a week.
Electrical lighting has allowed our bodies to move away from this natural clock, with two results in particular. Your circadian rhythm has probably diverged from that of a lot of people around you (you like to go to bed at 10 and wake up at 7; your roommate stays up til 2 a.m. and rolls out of bed around noon). Those preferences are genetically determined but also influenced by the environment. The second result is that whenever you do get up, your brain probably feels like mush (cities are making us groggy!). Scientists call this phenomenon "sleep inertia."
The campers in this study didn't exactly fall asleep with sunset and wake up at sun rise. The melatonin levels in their bodies began to rise – preparing them for sleep – around sunset. This happens during the body's internal sense of nighttime, and most of us require about two hours after our melatonin starts to rise before we're fully ready for sleep. In the morning, melatonin levels start to fall before we wake up, preparing our bodies for wakefulness.
"What we find in modern electrical lighting patterns that we are exposed to is that those melatonin levels are still high for a couple of hours, or an hour at least after we wake up," Wright says. Because we've synched out bodies to late-night TV instead of the natural environment, we often aren't really ready to wake up when it's time to go to work.
It might seem as if you're exposed to more light now than you would be in the woods sitting under fluorescent lightbulbs all day. But the quality and intensity of artificial light is different from sunlight (it also doesn't change over the course of a day). And because electrical light has made it possible to spend so much time indoors, we're simultaneously exposed to significantly less natural sunlight.
"Anything we can do to bring in more natural light we think will help keep our clocks timed earlier," Wright says. This means you should spend more time outdoors and open your blinds (OK, this is the opposite advice you get to conserve energy costs in the summer). You could take walks in the morning and breaks from the office (although walking around cavernous Manhattan may not be quite as helpful). Conversely, dimming the lights in your house at night would help, as would dimming the intensity of all those electronic screens you stare at as you lie in bed. Architects, too, could design buildings that allow better natural sunlight.
There is obviously a lot of middle ground between the remote Rocky Mountains and a city that prides itself on never falling asleep. So is your clock closer to the one nature wants you to have in the suburbs? The exurbs? Wright says some previous research has found that brighter, more urban environments are associated with later circadian clocks than some of these other places. But it's also hard to isolate what's keeping people up: the light, the noise, something else?
Another way of interpreting this research, if you're someone who periodically needs to flee the city for a mountainside, is that this is scientific evidence for why nature sometimes feels so much more restorative than the city.