Circadian rhythms are biological cycles of approximately 24 hours that are either passive responses to exogenous environmental changes or controlled by internal mechanisms known as biological clocks.
Our sleep-wake behaviour is controlled by a free running clock with a cycle of around 25 hours that is entrained by a zeitgeber. In the case of sleep-wake behaviour, this adjusts the endogenous rhythm of the cycle to keep it at 24 hours to match the natural regular variation in light and dark.
Light exposure is known to reset the biological clock, disrupting the cycle by either delaying or curtailing sleep depending on when in the sleep phase such disruption occurs. The modern environment in which we live, with its plentiful sources of artificial light and shade, is thought to be responsible for such disturbances that either reduce or extend the time we spend sleeping.
However, not all light at night is unnatural. The moon is adequately bright to assume that its presence could play a role in modulating our sleep-wake cycle. What is controversial, however, is whether the moon’s cycle, not just its attendance as a light source, may also affect our sleep-wake behaviours. Now a study claims to have found that the length of time slept is modulated by the lunar cycle.
Research led by the University of Washington tracked the sleep patterns over the course of a lunar month of participants from rural Toba/Qom communities, with little or no access to electricity, as well as urbanised Toba/Qom communities and post-industrialized American communities, both with 24/7 access to electricity.
In the days before a full moon, when a waxing moon provides moonlight that is brighter than light at dusk, the length of time spent sleeping decreased and the onset of sleep was delayed. These results were found irrespective of locale, however the effect was of greater significance in those communities without 24 hour access to electricity but still present in those with access.
The results would suggest that human sleep synchronises with the lunar cycle regardless of the level of urbanisation an individual may find themselves in. The researchers proposed an evolutionary explanation for their findings, saying that during times when moonlight is bright enough to see, it is beneficial for pre-industrialised societies to remain active. Indeed, interviews with those Toba/Qom communities living in rural areas suggest nights accompanied by bright moonlight are associated with greater activity including more fishing, hunting, and socialising than those where the moonlight is not so bright.
Although the study was not able to provide a conclusive explanation to why the effects seen arise in the first place, it is able to provide strong evidence for the lunar cycle’s role in sleep modulation and a possible interpretation of this evidence.