For centuries, the full moon has been linked in popular imagination with restless nights, sudden mood changes, strange behaviour, and even myths of werewolves. Across cultures, the lunar cycle has carried associations with human health and psychology. But while these ideas remain deeply embedded in folklore, modern research has found little evidence to support them.
Writers, physicians, and philosophers through the ages considered the moon a powerful force over the human mind and body. The English jurist William Blackstone once suggested that mental states varied with lunar phases. Civilisations of Greece and Rome also recorded observations that linked phases of the moon with changes in health and emotion.
In the 1970s, psychiatrist Arnold Lieber developed the theory that the moon might influence “biological tides” within the human body, in a way similar to its effects on the oceans, potentially altering mood, aggression, or behaviour. Yet subsequent research, as reported by Healthline, challenged these claims. Today, scientists note that the clearest evidence of lunar influence remains in marine life: corals and certain species of fish reproduce in synchrony with the lunar cycle.
Research has suggested that sleep may be subtly influenced by moonlight. According to Medical News Today, people living in both rural and Indigenous communities, as well as urban populations, tended to fall asleep later and sleep for shorter periods in the nights preceding a full moon. Some researchers interpret this as an evolutionary response, with brighter natural light extending activity into the evening.
The Cleveland Clinic explained that although the full moon increased natural light exposure, the illumination was far weaker than artificial sources such as electric streetlights or the glow from mobile phone screens. Consequently, the practical effect of moonlight on sleep was considered limited in modern settings.
The body’s circadian rhythm governs the timing of sleep, hormone release, and general health. Disruptions in this cycle are known to worsen conditions such as anxiety, depression, and bipolar disorder. Yet, despite long-held suspicions of a lunar connection, the evidence is slim.
Psychologist Susan Albers, quoted by the Cleveland Clinic, said that anecdotal claims of psychiatric wards filling up during full moons were not supported by consistent data. A review cited by Healthline in 2019 examined nearly 18,000 medical records and found no significant correlation between the lunar cycle and admissions or psychiatric outcomes.
There were rare exceptions. A small number of patients with rapid-cycling bipolar disorder had mood shifts that aligned with lunar phases. But such cases were considered outliers rather than evidence of a wider phenomenon.
Because of the moon’s gravitational pull, some researchers explored whether it might also affect the cardiovascular system. Healthline reported studies involving people with type 2 diabetes that observed slight changes in heart rate and blood sugar levels but no consistent alterations in blood pressure.
On reproduction, Medical News Today noted that while some menstrual cycles occasionally appeared to line up with the lunar calendar, comprehensive studies found no reliable pattern. Similarly, claims that childbirth was more frequent during full moons were contradicted by conflicting data, leaving no firm conclusion.
Experts highlighted the importance of psychological expectation in shaping perceptions of lunar influence. The Cleveland Clinic suggested that phenomena such as illusory correlation and self-fulfilling prophecy often explained why people believed the moon affected their behaviour. If an individual expected irritability or restlessness during a full moon, they might notice or amplify those feelings, reinforcing the belief.
While the moon has inspired centuries of folklore and speculation about its effects on human behaviour, modern science has found little proof of direct influence. Sleep may be slightly affected in the days before a full moon, but impacts on mood, reproduction, or mental health remain minimal.
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