written by Tara Haelle

I’m back! My apologies for the long absence — the summer time travel schedule becomes quite hectic, especially when combined with work deadlines. In addition, our family also took a one-week vacation to Costa Rica the week before last, during which I outlawed myself from the Internet (unless needed for hotel bookings or travel-related utility), including email, Facebook and blogging. (Then, of course, I spent the subsequent week recovering and catching up with work!)

But ironically, the first thing I’m writing about relates to our week in Costa Rica — the way our circadian rhythms are influenced by natural light-dark cycles. A study published in the journal Current Biology yesterday describes an interesting experiment whose results are quite timely in light of what I experienced while in Central America and what a friend recently told me about her experience now that her family has gotten rid of the television.

First, some background: both the now-TV-less friend and I are natural night owls. The technical terms is that we favor “eveningness,” and people with severe eveningness may have delayed sleep phase disorder. Given that my 3 am pretty much always naturally feels like others’ 11 pm for going to bed, and my 7 am pretty much always feels like others’ 3 am for waking up (I’m truly worthless before 9am, regardless of my bedtime), I strongly suspect I have delayed sleep phase disorder.

Except when I’m camping. Or in Costa Rica. I found that on our vacation, I naturally felt like going to bed most nights between 10:30 pm and midnight. And although I set an alarm for our exceptionally early mornings (like an ill-fated 6 am fishing trip or a 5 am shuttle to the airport), I generally felt fine getting up between 7:30 and 8:30 am, even though I normally D-R-A-G myself from bed between 9:30 and 10:30 am in real life. Meanwhile, my friend Anna’s toddler recently sent their TV to the grave, and they decided not to replace it. She told me she’s feeling tired earlier in the night now, falling asleep more easily, sleeping less restlessly (she often suffered insomnia or took forever to fall asleep) and feeling more refreshed in the morning. She believes — and there is research to back this up — that the light from the television had been disrupting her sleep cycles.

Which brings up to the study today that looked at exactly that: the influence of natural light-dark cycles on people’s biological clocks, compared to the influence of artificial light.

Circadian rhythms, or humans’ internal biological clocks, determine when we naturally go to sleep or wake up. When they’re out of whack, it may indicate a circadian rhythm disorder. Or, it might just indicate that we live in the modern world that has been revolutionized by the ubiquity of artificial electric light, especially since “the 1930s when electrical power grids in North America and Europe provided electricity to power electrical lighting for the masses, permitting humans to spend more time being active in indoor constructed environments,” the study authors noted.

Our clocks are “reset” by light, and while sunlight has the strongest effect, any kind of light can jack with our cycles. One of the hormones involved in circadian rhythms is melatonin. Its onset induces the sleepiness that makes us want to crawl into bed; its offset allows us to greet the world the next morning. In this study, researchers at the University of Colorado at Boulder studied six men and two women (average age 30) during a two-week experiment in which they compared participants’ melatonin onset/offset, bedtime, wake-up time, hours of sleep, sleep efficiency and light exposure in two different environments.

During the first week, the eight participants went about their normal daily lives in their home environments, which contained the same artificial lighting that dominates the life of most people in the world today where electricity is widespread. During the second week, the eight went camping with no flashlights, no cell phones, no iPads, no lanterns and no other kind of artificial light (though they did have campfires).

Several of the findings were not necessarily surprising, but I was still struck by them, including the fact that the participants were exposed to more than four times as much light during the camping week. During their waking hours in “regular life,” the participants were exposed to an average 979 lux, which the authors note is actually higher than typical, perhaps because the participants were outdoorsy types who lived in the sunny Rocky Mountains of Colorado. While camping, their daily average light exposure during waking hours was a whopping 4487 lux.

More importantly for the body’s clock, however, is the light the participants were exposed to in the first two hours after waking up. Light exposure during those two hours is the most crucial for “resetting” the body’s clock, much like winding it up for the next 24-hour cycle and, in a sense, telling your body how much time should pass before you should go to sleep again. In the artificial lighting environments of everyday life, the participants were exposed to an average 934 lux in the first two hours after awakening. While camping, their morning light exposure was an average 3074 lux.

There was, however, one time frame during which the participants were exposed to more light in their everyday, artificially lit environment than in natural light: between sunset and bedtime. During this time, the participants were exposed to about 21 lux in their everyday lives and about 8 lux while camping. Again, this is significant because artificial light during this time period is the most influential on circadian clock delays. While the morning light winds your clock up, light during the nighttime before going to bed slows it down, telling your body it’s… not… quite… time… for… bed… yet… and so you stay up later than you would have if you’d stayed on track from the morning reset. (Hence my friend Anne’s observation that she was appropriately tired at bedtime and falling asleep better when she was no longer watching TV so close to bedtime.)

In fact, the researchers also found that the individual differences among the participants in terms of their morningness or eveningness became less pronounced when they were camping. The night owls became a bit more like early birds, even though their average hours of total sleep and their average sleep efficiency (the number of minutes of sleep divided by the number of minutes in bed) remained similar during both weeks. During their week of regular life, the participants slept an average 6.7 hours a night with 87.6% efficiency ; while camping, it was 6.8 hours with 87% efficiency.

Of course, “social behaviors, removal of electrical lighting and increased physical activity” while camping could have all played a part in when the campers went to bed. But the effects of natural sunlight made a difference too, “resulting in larger circadian advances for those with later chronotypes.” In plain language, those who had typically stayed up later saw bigger changes in their (earlier) bedtimes when camping because their biological clocks were getting or reset (advanced) earlier.

The researchers observed that night owls’ biological clocks tend to run even later than early birds’ clocks do when exposed to less sunlight. This may seem obvious: they’re night owls, so of course they see less daylight. But the researchers suggested the direction runs the other way: it’s because they get less daylight that they’re night owls. And the daylight may worsen sleep/circadian problems like delayed sleep phase and jetlag, they wrote. (Hence the use of sun lamps to treat delayed sleep phase disorder.)

And the bottom line to all this is that — surprise! — humans are like any other animal in how our bodies react to sunlight cycles. The researchers wrote, “Our findings demonstrate a fundamental physiological principle of human circadian timing — internal biological time under natural light-dark conditions tightly synchronizes to environmental time, and in this regard, humans are comparable to other animals.”

While fascinating, most of these findings simply confirmed what biologists and sleep doctors already knew about how circadian rhythms were influenced by artificial light. What I found more compelling were the changes in the participants’ melatonin onset and offset during the two different weeks.

In their everyday, artificially lit lives, the participants’ melatonin onset was about two hours before they went to sleep, which was typically around 12:30am. The melatonin hit its midpoint during the second half of “solar night,” which is the full period from sunset to sunrise. The melatonin switched off about 8 am — more than an hour after participants had woken up.

After spending a week camping, however, the melatonin pattern shifted two hours earlier: onset about sunset, midpoint exactly in the middle of solar night and offset just after sunrise — about 50 minutes before participants woke up.

Aha! That groggy morning drudge out of bed to make coffee, during which it takes you more than an hour to really “wake up” once you’re out of bed? Yea, your melatonin is still going strong, baby. But while camping, the melatonin offset occurs nearly an hour before you open your eyes to greet the world. Hence the reason I felt refreshed each morning on my Costa Rica trip, when I was more or less following the natural daytime/nighttime cues for my sleep. I also wasn’t using my computer late into the evening as I usually do (and am right now, actually). The one night I remember staying up late in Costa Rica, I was actually playing a stupid game on my iPhone.

Now, before I get too carried away extrapolating this study to you and me, I’ll mention again that it only involved eight participants, which is hardly more than a case study. Yet these findings support much of what past circadian rhythm research has already revealed, as the authors discuss. Our cognitive performance is lowest at the “circadian low point in brain arousal,” which research has shown occurs about two hours after people habitually wake up. But that sleepiness we experience after waking up might exist primarily because we now live in artificially lit environments. Potentially, “if human circadian and sleep timing was in synchrony with the natural light-dark cycle, the circadian low point in brain arousal would move to before the end of the sleep episode, making it easier to awaken in the morning.”

Aside from the small number of people in the study, another limitation is that it was conducted in sunny Colorado during July. Folks living through the long, dark Alaskan winters or the endlessly sunny Norwegian summers would get too much or too little sleep if their bodies followed the cues of the natural light-dark cycles, and this study does not address what the body does do in those circumstances (nor in places like rainy Seattle, where the sunlight is muted through the clouds much of the time).

But clearly, the study shows that, yes, electrical light messes with our body clocks, having “altered human circadian physiology, leading to a major change in the timing of our sleep and wakefulness.”

But the findings do offer suggestions for people trying to straighten out their biological clocks: “exposure to the natural light-dark cycle may help to obtain a desired earlier timing of the circadian clock and sleep for patients with delayed sleep phase. In addition, increased exposure to sunlight combined with a fixed earlier sleep schedule may help adolescents and young adults who need to awaken early for school and work to maintain earlier sleep and wake timings.” So, if your clock isn’t quite ticking right, or you’re coming off shift work, or you just need to get your butt out of bed earlier, get thee outside to soak up the sun. It might help “reduce the physiological, cognitive and health consequences of circadian disruption.”

Now if only I could get it to work with a newborn, I would so totally go camping in my backyard for those first two months after birth!