The full moon has been blamed for sleeplessness for centuries, usually with more folklore than evidence behind it. But one laboratory study from the University of Basel made the old claim harder to dismiss outright. In a controlled sleep experiment, participants around the full moon took longer to fall asleep, had less deep sleep and showed lower evening melatonin, even though the moon itself was not visible to them.
The finding comes from a 2013 Current Biology paper by Christian Cajochen and colleagues. The researchers did not set out to test moonlight in the usual sense. They retrospectively examined sleep recordings, hormone measures and electroencephalogram data collected under tightly controlled circadian laboratory conditions, then compared them with lunar phase.
That detail is the reason the study became famous. The result was not simply that people sleep badly when moonlight enters a bedroom window. The volunteers were studied in a laboratory without ordinary time cues, and the authors wrote that neither the volunteers nor the investigators were aware during or after the original study that the data would later be analyzed by lunar phase.
What changed near full moon
The reported effects were modest in clock time but clear enough to attract attention. Around the full moon, participants took about five minutes longer to fall asleep. Their total sleep time, measured by EEG, was about 20 minutes shorter. Their EEG delta activity during non-rapid-eye-movement sleep, a marker associated with deep sleep, was about 30% lower.
The same Current Biology paper also reported lower subjective sleep quality and diminished endogenous melatonin levels around the full moon. Melatonin is a hormone made by the body, mainly at night, and is involved in the timing of sleep and circadian rhythms. The key point is that this was measured inside a controlled laboratory, not inferred from whether participants said the moon felt bright.
There were only 33 volunteers, so this was not a huge population study. It was also a retrospective analysis, meaning the lunar question was asked after the original sleep data had already been collected. Those limits matter. The study is best read as evidence of a possible circalunar signal in human sleep physiology, not as proof that the full moon reliably disrupts everyone.
Why the lab setting matters
If a person sleeps worse on a bright night, the easiest explanation is light. Moonlight can alter nighttime visibility, behavior, activity and the timing of going to bed. In ordinary life, that makes it difficult to know whether the moon is affecting biology directly or simply changing the environment.
Cajochen and colleagues tried to remove those obvious explanations by using existing data from a circadian laboratory protocol. The paper’s summary says the study excluded confounders such as increased nighttime light and expectation about lunar influence. That does not prove a mechanism. It does make the result stranger, because the simplest “they saw the moon” explanation is not available.
The authors suggested that the pattern could reflect circalunar rhythmicity, a roughly 29.5-day rhythm tied to the lunar cycle. Such rhythms are well established in some marine organisms, but whether humans retain a meaningful version is a much harder question. The study did not identify the biological pathway. It measured a pattern and left the mechanism open.
Why scientists stayed cautious
The history of claimed lunar effects is full of false starts. Human behavior, births, hospital visits and mood have all been linked to the moon in popular culture, and many such claims have not survived careful statistical testing. Sleep is more plausible than many of those claims because light and circadian biology are real, measurable factors, but plausibility is not the same as certainty.
After the Basel paper, other researchers pushed back. A 2014 Current Biology article titled “Lunar cycle effects on sleep and the file drawer problem” analyzed larger sleep datasets and argued that the lunar effect was not supported there. Its title pointed to a familiar scientific worry: positive findings are more likely to be noticed, published and remembered than null findings sitting unseen in a file drawer.
There were earlier hints in the other direction, too. A 2006 Journal of Sleep Research study found that subjective sleep duration in a small Swiss sample varied with lunar phase, with shorter reported sleep near the full moon. But that study was also originally designed for another purpose, and its authors said the association needed confirmation.
Later field studies complicated the picture
More recent work has kept the question alive without making it simple. A 2021 Science Advances study called “Moonstruck sleep” examined sleep timing under field conditions and reported that people tended to sleep less and go to bed later in the nights before a full moon. The pattern appeared in communities with very different access to electric light, although the size of the effect varied.
That field result fits a different explanation from the Basel laboratory study. In real-world settings, moonlight before a full moon could extend evening activity, especially where artificial light is limited. In a darkened laboratory, however, the Basel result cannot be reduced to a person staying outside longer because the night is brighter.
There are also studies that find very small effects or no meaningful effect at all. This is why a careful article should not say “the full moon ruins sleep.” A better reading is that some datasets have found lunar-cycle associations with sleep timing or sleep architecture, while others have not. The evidence is intriguing, uneven and still mechanistically uncertain.
What melatonin adds
The melatonin finding is one of the reasons the Basel study still stands out. Sleep diaries can be biased by memory or expectation, and even sleep timing can shift for social reasons. Hormone data add a more physiological layer. Lower evening melatonin around the full moon, if real, suggests the pattern was not only a matter of subjective complaint.
Still, melatonin is sensitive to many factors, especially light exposure and circadian timing. A small retrospective laboratory study cannot show whether a lunar rhythm was driving the change, whether the finding depended on the specific sample or whether some unmeasured factor lined up with lunar phase by chance.
That is the tension that makes the result interesting. The study was controlled enough to rule out some obvious explanations, but small enough that replication matters a great deal. It sits in the scientific middle ground: too carefully measured to dismiss as superstition, too limited to treat as settled biology.
The most accurate conclusion is therefore modest. In one University of Basel dataset, sleep changed around the full moon even when participants could not see the moon or know its phase. They fell asleep a little later, slept a little less, spent less time in the deepest sleep signal and produced less melatonin. Whether that reflects a real human circalunar rhythm, a fragile statistical signal or something in between remains an open question.