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.