03/11/2025
How's your inner lunar clock?
There's a full moon on November 5th...
HUMANS HAVE AN INTERNAL LUNAR CLOCK - BUT LIGHT POLLUTION IS DISRUPTING IT
Most animals, including humans, carry an internal lunar clock, tuned to the 29.5-day rhythm of the Moon. It guides sleep, reproduction and migration of many species. But in the age of artificial light, that ancient signal is fading - washed out by the glow of cities, screens and satellites.
Plenty of research suggests the lunar cycle influences human sleep. A 2021 study found that in Toba Indigenous communities in Argentina, people went to bed 30-80 minutes later and slept 20-90 minutes less in the three-to-five nights before the full Moon.
Similar, though weaker, patterns appeared among more than 400 Seattle students in the same study, even amid the city’s heavy light pollution. This suggests that electric light may dampen but not erase this lunar effect.
The researchers found that sleep patterns varied not only with the full Moon phase, but also with the new- and half-Moon phases. This 15-day rhythm may reflect the influence of the Moon’s changing gravitational pull, which peaks twice each month, during both the full and new Moons, when Sun, Earth and Moon align. Such gravitational cycles could subtly affect biological rhythms alongside light-related cues.
Laboratory studies have supported these findings. In a 2013 experiment, during the full Moon phase participants took about five minutes longer to fall asleep, slept 20 minutes less, and secreted less melatonin (a hormone that helps regulate the sleep-wake cycle). They also showed a 30% reduction in EEG slow-wave brain activity - an indicator of deep sleep.
Perhaps the most striking evidence of a lunar rhythm in humans comes from a study just published analysing long-term menstrual records of 176 women across Europe and the US.
Before around 2010 - when LED lighting and smartphone use became widespread - many women’s menstrual cycles tended to begin around the full Moon or new Moon phases. Afterwards, that synchrony largely vanished, persisting only in January, when the Moon-Sun-Earth gravitational effects are strongest.
Losing the slow, monthly cue of moonlight may mean that our internal clocks now run in a flatter temporal landscape, with fewer natural fluctuations to anchor them. Previous psychological research has found that disconnection from nature can warp our sense of time.
The lunar clock still ticks within us - faint but measurable. It shapes tides, sleep and the rhythms of countless species. Yet as the night sky brightens, we risk losing not only the stars, but the quiet cadence that once linked life on Earth to the turning of the Moon.
