How does the Moon affect sleep patterns?

In 1969 man landed on the Moon. But since then it has lost none of its mystery. From Lucian's fictional account of travelling to the Moon in the second century AD to "An American Werewolf in London", a 1981 horror film, the Moon continually inspires and puzzles. Now scientists have discovered that people sleep more fitfully on nights when there is a full Moon. How does the Moon affect sleep patterns?

The transformative nature of the Moon was long suspected. It can be heard in words such as "moonstruck" and "lunatic", or glimpsed in fiction. The lycanthrope, a sinister figure who turned into a wolf at full Moon, has dogged horror stories. And even outside such folk tales many scientists have suspected that the Moon may affect the way that people sleep at night.

A study published in Current Biology by Christian Cajochen and colleagues at the Centre for Chronobiology at the University of Basel gives weight to this theory. It was based on data from a study on body clocks and sleep patterns conducted between 2000 and 2003. In that experiment, volunteers were shut away from daylight (and thus moonlight) and their sleep patterns monitored. Neither the participants nor the organisers knew, at the time, that the data would later be used to examine the effect of the Moon, making it a perfect double-blind experiment. When Mr Cajochen and his colleagues compared the data with the lunar cycle, a relationship emerged.

The volunteers slept, on average, 20 minutes less around the time of the full Moon. It took them five minutes longer to get to sleep, and their delta activity (which measures how deeply they slept) was 30% lower than at other times. Melatonin, a sleep-related hormone, was reduced. They reported that they had not slept as well as usual. And none of this was connected, in female participants, with menstrual cycles. The results show that the phase of the Moon affects human sleep patterns, even when the person in question cannot see the Moon.

The researchers believe that, rather than a tidal effect of the Moon on sleep patterns, they have discovered an additional hand on the body's clockface. As with the endogenous daily cycle entrained to the sun, a monthly cycle connected to the Moon exists through a long period of unconscious observation by the body. But it remains something of a puzzle as to why humans are affected by the Moon. Knowledge of the tides is not important to humans in the way that it is to marine iguanas, say, which studies have also found to have lunar cycles. One theory is that lighter sleeping patterns help to guard humans against predators that are able to see more clearly under moonlight. Man's relationship with the Moon continues.