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Key points
- New research demonstrates that moonlight can affect the circadian rhythm.
- Researchers have found a pattern of the longest times to fall asleep and the shortest sleep periods occurring a few nights before the full moon.
- Subtle environmental factors can have a significant impact on our sleep.
We are very aware of the impact of sunlight on our day/night cycle. Light is the most powerful environmental factor that daily resets the internal circadian (24-hour) clock along with eating, activity, and social interaction. As spring approaches and the period of daylight is increasing, many people are feeling better, especially after this long, cold, and frightening winter of COVID-19.
Of course, other sources of light also affect the circadian rhythm. Indoor lighting provides the potential to have bright light around us long after the sun has set. As a society, our sleep cycles have been affected by this, and most people stay up later than people did in the premodern world. Prior to the invention of electric lighting, there were few sources of bright light after sunset. Perhaps there might have been a brilliant flash of lightning or an especially bright bonfire. But venturing out into a dark night could be very dangerous. Luckily, there was a light source that could have provided some light to allow a measure of safety. That source of light was the moon.
I have, on more than one occasion, heard people say that they have an especially hard time sleeping during the full moon phase of the lunar cycle. It is hard to evaluate the significance of these reports. Is it just a myth or could there be something to this? The effect of moonlight on the circadian rhythm of certain animals has been established, but this has been more difficult to do with humans. Possible links between sleep and the lunar cycle have been investigated in the past with inconclusive results. Some but not all studies have shown an association.
A new study by Casiraghi et al (2021), has attempted to improve the methodology of past studies that have investigated the effects of moonlight on sleep. They looked at four different communities with varying access to electric lighting. Three were indigenous communities in Argentina. Of these, two were rural and one was urban. The fourth was an urban, post-industrial community in the U.S.
The three indigenous communities were among the Toba/Qom of Argentina. The urban, postindustrial one was composed of American college students. There were 98 participants in the study from Argentina. Fifty-six percent of the sample were women. The average age was 24.1 years with a range of 12-75. One of these communities was an urban environment with full access to electrical lighting indoors and with street lighting out of doors. A second community was rural with limited access to electrical lighting such as one light bulb per room and no outdoor lighting. The third community was a rural community without access to electrical lighting in homes or out of doors. These communities have different lifestyles but share a common history, language, and customs. Data were collected over a three-year period of time in the spring (in the Southern Hemisphere this is September to November) to keep environmental conditions as stable as possible.
The college sample was composed of University of Washington students. The average age was 21.5 with a range of 18-38 and 62% were women. These data were also collected during a three-year period.
Sleep was assessed using movement logging watches and by logs completed by the participants. Analysis of the movement data allowed for the determination of sleep and wake periods. These were then statistically compared to astronomical data for the sun and moon phases taken from NASA’s Jet Propulsion Laboratory HORIZONS Web-Interface.
The researchers found a pattern that has been previously identified with regard to the effect of electric lighting. Time of sleep onset was later and the length of sleep shorter the more lighting was available. This was noticeable across the groups with the urban American sample the most affected, as would be expected. They also found a consistent pattern of the longest times to fall asleep and the shortest sleep periods occurring on the three to five nights before the full moon. This was observed in all four communities.
Moonrise occurs about 50 minutes later each day and so moonlight becomes less available in the early evening hours following the full moon and so its impact on falling asleep is likely decreased. Individuals across the four groups showed a decrease of 20 to greater than 90 minutes of sleep during the days before the full moon. Along with this, there was an increase in the amount of time it took to fall asleep. The increase ranged from 30 to 80 minutes. There was an insignificant effect on wake times.
Casiraghi et al (2021) point out that moonlight is sufficiently bright to permit outdoor activities and thus would have an inhibiting effect on falling asleep, although this degree of light would be dim enough to not wake someone who has already fallen asleep. Since people usually fall asleep sometime after sunset and rarely get up before dawn, the effects of bright moonlight would be less when present later in the night as compared to when the brightest moonlight is earlier in the evening when people are still up, as in the lead up to the full moon. These findings suggest that moonlight did affect the sleep of people in preindustrial societies and shortened sleep on nights that were brighter. They also indicate that people in postindustrial societies also experience these effects.
This study demonstrates the significant impact that subtle environmental factors can have on our sleep. If even a few nights of moonlight can affect the amount of sleep we get, we can imagine the impact that nightly use of bright light-emitting electronic devices has on those of us who are nightly users.
References
Casiraghi, L., Spiousas, I., Dunster, G. P., McGlothlen, K.. Fernández-Duque, E., Valeggia, C., de la Iglesia, H. O. (2021). Moonstruck sleep: Synchronization of human sleep with the moon cycle under field conditions. Science Advances, 7 (5), 27 Jan 2021, eabe0465, DOI: 10.1126/sciadv.abe0465.
