Moonstruck sleep: Synchronization of human sleep with the moon cycle under field conditions

Abstract

Before the availability of artificial light, moonlight was the only source of light sufficient to stimulate nighttime activity; still, evidence for the modulation of sleep timing by lunar phases is controversial. Here, we use wrist actimetry to show a clear synchronization of nocturnal sleep timing with the lunar cycle in participants living in environments that range from a rural setting with and without access to electricity in indigenous Toba/Qom communities in Argentina to a highly urbanized postindustrial setting in the United States. Our results show that sleep starts later and is shorter on the nights before the full moon when moonlight is available during the hours following dusk. Our data suggest that moonlight likely stimulated nocturnal activity and inhibited sleep in preindustrial communities and that access to artificial light may emulate the ancestral effect of early-night moonlight.

Fig. 1. Sleep timing changes through the moon cycle.

(A and B) Double plots of the average duration and onset of sleep in the Toba/Qom population across the moon cycle expressed as average z scores (±SEM; N = 69 participants). Solid lines represent the best fit of the complete dataset to sinusoidal curves with a 30-day period from a nonlinear least squares fit (see Materials and Methods), and the vertical dashed lines indicate the trough of sleep duration (i.e., the shorter sleep events) and the acrophase of sleep onset (i.e., the latest sleeping times). Best-fit equations are indicated for each variable. Fitted sine wave amplitudes mean and 95% confidence intervals (CIs): duration: 0.31 [0.25 to 0.37]; onset: 0.46 [0.40 to 0.51]. (C and D) Double plots of the average values (±SEM) of duration (in minutes) and onset of sleep (in minutes after the astronomical dusk) by community. Solid lines represent best fits for each community data subset. Fitted sine wave amplitudes mean and 95% CIs: duration: Ru-NL, 8.8 [4.9 to 12.8]; Ru-LL, 7.5 [4.0 to 11.0]; Ur, 9.4 [4.6 to 14.2]; onset: Ru-NL, 10.0 [6.2 to 13.7]; Ru-LL, 12.1 [9.1 to 15.1]; Ur, 6.4 [2.5 to 10.3]. Amplitude and phase parameters for all fits are summarized in table S9. Individual data series for participants with records for at least 80% of the moon cycle were filtered through moving average with a window of 7 days before summarizing the data. Number of participants: Ru-LL, 20; Ru-NL, 23; Ur, 26.

Fig. 2. Individual patterns of sleep timing across the moon cycle.

Double plots of (A) sleep duration and (B) onset of sleep, expressed as z scores, of the 35 best-fitting participants (as evaluated by the standard error of regressions, S) in the study for each variable. Dots indicate the data on a given night in the cycle, and colored lines represent the best fit of a sine wave with a 30-day period through a nonlinear least squares approach. Individual data series for participants with records for at least 80% of the moon cycle were filtered through moving average with a window of 7 days before summarizing the data. The numbers on the bottom of each plot identify the participant (left) and the S value for the fit (right). The data for the complete set of participants are presented in figs. S3 and S4.

Fig. 3. Analysis of changes in sleep variables according to the No-ML (no moonlight) and F-ML (full moonlight) phases.

(A) Duration of sleep (hours), (B) sleep onset (hours after dusk), and (C) sleep offset (hours after dawn). Smaller dots represent each individual’s mean, while the bigger dots represent the means for each community and are connected by color lines. Vertical black error bars represent the 95% CI of the mean. The reported P values correspond to the main effect of moonlight for the type III analysis of variance (ANOVA); Cohen’s d values are for Tukey contrasts between moonlight phases in each community in the linear mixed-effects models. Only participants with at least four nights recorded in each phase were considered. Number of participants: Ru-NL, 25; Ru-LL, 25; and Ur, 32.

Fig. 4. Association of sleep duration and onset with the moon cycle in a highly urban setting.

Double plots of (A) sleep duration and (B) sleep onset expressed as z scores (±SEM) on weeknights recorded on 463 college students in different quarters from 2015 to 2018. The differences between individual data points and the mean values in each season were calculated for over ~4300 sleep events. The solid lines represent the best fits to sine waves with a 30-day period to the data from nonlinear least squares fits (see Materials and Methods); wave equations are printed at the bottom left. The dashed vertical lines indicate the phase of shorter (A) and later (B) sleep events. Fitted sine wave amplitudes and 95% CI: duration, 0.34 [0.13 to 0.55]; onset, 0.32 [0.10 to 0.53]. The average data summaries were filtered through a moving average with a window of 7 days. Participants/sleep events per quarter: spring, 173/1729; summer, 66/619; fall, 136/1240; and winter, 88/796.