By Kelly Servick
Our internal clocks are drifting out of sync, and indoor lighting may be to blame. A new study suggests that just a few days in the great outdoors puts us back in tune with the solar cycle, and reconnecting with the sun could make us less drowsy.
Electricity has given us the freedom to choose our bedtimes; staying up after dark is as easy as flipping a light switch. But we pay a price for this luxury, says integrative physiologist Kenneth Wright, of the University of Colorado, who led the new study. People with later bedtimes and wake times are exposed to more artificial light and less sunlight, he says, which means their bodies aren't getting the natural cues humans once relied on.
To understand how falling out of sync with the sun changes our body's internal clock - or circadian rhythm - sleep researchers look to the timekeeping mechanisms in the brain, particularly how we regulate the hormone melatonin. Released about two hours before sleep, melatonin makes us feel drowsy as we wind down for rest, Wright says. It then decreases as we become alert in the morning. The mechanisms driving our clock are complex and hard to measure, but the daily spike and drop in melatonin are like its chimes. "Melatonin tells us what time it is in the body," Wright says.
And when we keep strange schedules, our melatonin goes haywire. Turning lights on at night can delay melatonin release and shift the timing of our internal clock, says sleep physiologist Derk-Jan Dijk, of the University of Surrey in England, who was not involved in the work. But it wasn't clear just what would happen in modern, electricity-adapted humans if all artificial light were suddenly taken away. "This is the first time that somebody has done the obvious but important experiment," he says.
Wright and his colleagues outfitted eight subjects with activity-tracking watches that carry light intensity detectors and motion sensors to keep tabs on sleep and wake times. For the first week, the participants went about their lives, spent mostly in artificially lit buildings. They then spent 24 hours in a lab, where the researchers periodically tested the melatonin levels in their saliva. In the second week, the group went camping in the Colorado Rockies, where they could sleep and wake up whenever they wanted but had no access to TV, cell phones or flashlights. Their world was illuminated only by sunlight and campfires. The group returned from their excursion for another stint of saliva sampling.
Data from the watches showed that subjects got about the same amount of sleep in the two settings. But the shift from artificial to natural light, which nearly quadrupled their total light exposure, also tinkered with their internal clocks. After camping, the subject's biological cycles had shifted to align with the sun. Their bodies released melatonin right at sunset - two hours earlier than under artificial light conditions - shut it off again just after sunrise, the team reported last week in Current Biology.
"When we expose ourselves to only natural light, we are in sync with that light-dark cycle quite strongly," Wright says.
The natural night owls in the group saw an especially dramatic shift in their melatonin cycle and became more similar to the early birds. The team suggests that artificial light had been exerting a particularly strong influence on the internal clocks of the night owls. The subjects weren't asked to report whether they felt less drowsy after the change in lighting.
Observing changes in human rhythms in a natural environment represents a "breakthrough," says Marie Dumont, a chronobiologist at the University of Montreal. "I think we forget most of the time that the knowledge that we have comes from laboratory and artificial conditions," she says. Dumont cautions, however, that few conclusions can be drawn from this small group of individuals. Changes in physical activity during the camping trip and the social interaction subjects had also likely influenced the retiming of their internal clocks, she says.
But the work may offer clues about the tiredness that plagues many night owls. Other studies have shown that our low point in alertness, when melatonin production is shutting off in the morning, tends to occur about two hours after awakening.
"We wake up, but then our clock still promotes sleepiness, and we don't feel well," explains Dijk, whose research group first described this unfortunate paradox. After the week of camping, participants' melatonin shutoff occurred before they awoke instead of after. Wright says that the discrepancy between our melatonin cycle and our sleep-wake cycle could account for our morning sleepiness, an explanation Dijk calls "an interesting suggestion" that needs more thorough study.
Because we're not going to abandon our electrified existence anytime soon, Wright says that certain habits can counteract our estrangement from the sun. He recommends letting plenty of light into your room in the morning, exposing yourself to more natural light throughout the day, and dimming the lighting in your home a couple hours before bed.