The study looked at official statistics for Greece, Cyprus, Italy, France and Spain.
Extreme heat and cold are associated with an increased risk of death.
"We speculate that another factor accounting for the lower mortality observed in September in the Mediterranean countries is the preceding summer vacations," write Dr. Matthew Falagas of the Alfa Institute of Biomedical Sciences in Greece and coauthors. The authors suggest the physiological effect of increased vitamin D synthesis combined with the stress-relieving benefits of time off may contribute to these lower mortality figures.
In North America and Sweden, August is the lowest month for mortality, in Japan it is July, and in Australia, it is March (the counterpart to September in the southern hemisphere).
The authors call for additional research to clarify the possible causes of these drops in mortality rates.