Full moon may mean less sleep

A full moon deprives people of sleep even when they are shielded from moonlight in a windowless lab, a new study suggests.

People snoozed less deeply within four nights of a full moon than during other parts of the lunar cycle, researchers report July 25 in Current Biology. The authors suggest that humans may have internal clocks that track the lunar cycle, much like circadian clocks that sync up with the rise and fall of the sun.

Christian Cajochen of the University of Basel in Switzerland and his colleagues reanalyzed sleep data they had collected over several years from 33 people who had each spent several days half-reclining in bed under constant dim light. Looking at only the second night of each participant’s stay, the researchers found that around the full moon, participants took about five extra minutes to nod off, slept for about 20 minutes less each night and slept less deeply.

The team was surprised to uncover the lunar rhythm, and Cajochen was initially reluctant to share the findings. “If you publish lunar stuff, you are going to be put in the ‘lunatic’ corner and not be considered a serious sleep researcher anymore,” he says.

But other scientists praise the work. “This was done under really controlled laboratory conditions,” says Kenneth Wright, a sleep researcher at the University of Colorado Boulder. It’s the first laboratory study to detect an influence of the moon on human sleep, he says.

The result needs explanation. The shifting gravitational effects of the moon are too weak to influence human bodies, Cajochen says. His hypothesis is that humans have an internal body clock synchronized to the phases of the moon.

An alternative is that participants’ light exposure before the study may have affected their circadian clocks enough to disturb their sleep in the lab. People’s circadian clocks are particularly sensitive to light at night, so extra moonlight before the study might be the simplest explanation for differences during it, says David Dinges, a sleep researcher at the University of Pennsylvania.

Scientists also aren’t sure why humans would have evolved lunar rhythms, though many marine organisms have lunar clocks to keep track of tides, Dinges says. “Stay tuned,” he says. “There’s going to be a lot more research to nail this down.”