New findings published in Science Advances suggest that human sleep cycles may vary in line with lunar cycles, regardless of whether individuals are in rural or urban environments.
When humans were hunter-gatherers, there was a fairly simple rule about bedtime. Once the moon had risen, sleep quickly followed (unless you wanted to be devoured by a lurking predator). That logic stuck hard and fast for thousands of years. That is until the advent of electricity permitted the invention of circadian clock-defying feats, such as clubbing (or, more recently, 3 AM Netflix binges).
A collaborative study between the University of Washington (UW), Yale University and a group from the National University of Quilmes’s Sensorimotor Dynamics Lab in Argentina, has shown that when it comes to moonlight and sleep, old habits die hard.
The team, led by UW Professor Horacio de la Inglesia, studied the sleep habits of 98 individuals, all from the indigenous Western Toba/ Qom community, a group who maintained a hunter-gatherer society until the arrival of Spanish colonizers in the 1500s. Now, the Western Toba/Qom are spread across a variety of locales. The researchers selected three groups – one who lived without electricity, one who had only limited access to electric light and a final group who had unfettered access to electricity. They asked whether, across these three groups, sleep duration and onset would vary depending on the lunar cycle, predicting that such effects would be limited only to those in settings without electricity. Their predictions proved far off the mark.
Regardless of the urbanization of their settings, participants in the trial – who had their sleep measured by actimetry bracelets – showed a uniform oscillation in their sleep pattern, with the shortest sleep and latest bedtime being recorded three to five days before the full moon. The authors write that this suggests the participants’ bodies were naturally preparing to be more active in line with brighter evenings.
This produced some powerful variation, with participants sleeping on average ~50 minutes longer at the peak of the oscillation.
The power of moonlight
The urban setting the authors initially chose, in Argentina’s northwestern Formosa Province, has a population of just 19,000. Could life in a sprawling city weaken the influence of the lunar cycle on sleep? To answer the question, the researchers looked at a further group, using data taken from 464 college students at UW whose sleep data was recorded as part of a separate study. The results from this group, who all lived in the unquestionably urban environment of (population: 776,000) also showed an adherence to the flex of the lunar cycle in their sleep habits.
The findings point to ancestral adaptions in our sleep-wake cycles, said UW postdoctoral researcher Leandro Casiraghi in a press release: “We hypothesize that the patterns we observed are an innate adaptation that allowed our ancestors to take advantage of this natural source of evening light that occurred at a specific time during the lunar cycle.”
Interestingly, there was no relationship found between time of awakening and moon phase – de la Inglesia that this is another dimension in which electricity’s impact on our circadian clocks mimics that of moonlight: “In general, artificial light disrupts our innate circadian clocks in specific ways: It makes us go to sleep later in the evening; it makes us sleep less. But generally we don’t use artificial light to ‘advance’ the morning, at least not willingly. Those are the same patterns we observed here with the phases of the moon.”
de la Iglesia’s findings add evidence to a field that has been split on the question of how the moon influences our sleep patterns. Some previous studies, even those analyzing the same data from the UW students, had failed to find a relationship between lunar phase and sleep duration. The team suggested in their paper that their results were able to uncover a relationship due to their long-term mapping of the lunar cycle, rather than using only direct comparisons of data from full, waxing/waning and new moon phases.
To clear up these conflicts, more studies will be needed; it is clear that there are a lot of questions to be answered. Casiraghi summed up how these questions could be answered “Future research should focus on how: Is it acting through our innate circadian clock? Or other signals that affect the timing of sleep? There is a lot to understand about this effect.”
Casiraghi L, Spiousas I, Dunster GP, et al. Moonstruck sleep: Synchronization of human sleep with the moon cycle under field conditions. Sci Adv. 2021;7(5):eabe0465. doi:10.1126/sciadv.abe0465