![]() Humans typically start their daily sleep bout some hours after dusk but rarely wake up before dawn, a pattern we also documented earlier among the Toba/Qom ( 3). The moon is responsible for several environmental cycles, but its lighting power during the night is arguably the most relevant cycle to humans in natural conditions. Thus, moon phases were associated with predictable and biologically relevant changes in daily sleep timing. Changes in the onset of sleep varied from 30 to 80 min (Ru-NL, 29 Ru-LL, 32 Ur, 32 ). ![]() Changes in each participant’s sleep duration across the lunar cycle ranged from 20 to more than 90 min and did not differ considerably between groups. The individual phases of sleep duration and onset showed a consistent clustering of the troughs of sleep duration and the peaks of sleep onset on the days before the full moon (Rayleigh z tests, mean phase in days before the full moon : duration, 2.8, P = 6 × 10 −4 onset: 3.3, P = 3 × 10 −7 fig. We fitted individual data to sine waves with a 30-day period through a nonlinear least squares approach and analyzed the parameters of the best-fitting participants (i.e., the three best quartiles according to the standard error of the regression, S n = 51). ![]() The modulation of sleep duration and onset across the moon cycle was evident at the individual level for most participants in the three communities ( Fig. ![]() Number of participants: Ru-LL, 20 Ru-NL, 23 Ur, 26. 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. Amplitude and phase parameters for all fits are summarized in table S9. Fitted sine wave amplitudes mean and 95% CIs: duration: Ru-NL, 8.8 Ru-LL, 7.5 Ur, 9.4 onset: Ru-NL, 10.0 Ru-LL, 12.1 Ur, 6.4. Solid lines represent best fits for each community data subset. ( 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. Fitted sine wave amplitudes mean and 95% confidence intervals (CIs): duration: 0.31 onset: 0.46. Best-fit equations are indicated for each variable. 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). ( 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). The controversy generated by these studies has underscored the need for longitudinal studies that can assess the potential effects of moon cycle on sleep ( 11). ![]() Some authors have argued against strong effects of moon phase on human behavior and biological rhythms ( 6– 8), but recent studies have reported that human sleep and cortical activity under strictly controlled laboratory conditions are synchronized with lunar phases ( 9, 10). However, whether the moon cycle can modulate human nocturnal activity and sleep remains a matter of controversy. Moonlight is so bright to the human eye that it is entirely reasonable to imagine that, in the absence of other sources of light, this source of nocturnal light could have had a role in modulating human nocturnal activity and sleep. While the sun is the most important source of light and synchronizer of circadian rhythms for almost all species, moonlight also modulates nocturnal activity in organisms ranging from invertebrate larvae to primates ( 5). ![]()
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