Next month, observers of the Muslim holiday of Ramadan will fast daily between sunrise and sunset, and feast thereafter, while scientists in a handful of labs around the world will examine what's happening inside their bodies.
Ramadan, it turns out, has become a useful phenomenon for researchers studying circadian rhythms - and what happens to the body when they are disrupted.
During Ramadan, Muslims eat and get more active just when their bodies are used to winding down, creating sleep disruptions, hormonal changes, and sometimes mood impacts.
"Their biological clocks are no longer in harmony with their watches," said Yvan Touitou, a chronobiologist at Pierre and Marie Curie University in Paris. "Ramadan is capable of desynchronizing people."
Touitou's research has illustrated that Ramadan can alter the usual circadian patterns of cortisol, a stress hormone, and testosterone, with sharper decreases of these hormones in the morning and later rises at night - though the impact of these rhythm disruptions is unclear.
The holiday also changes the schedule of the release of leptin, a hormone that regulates appetite and weight, and decreases the peak levels of melatonin, a hormone released at night to induce sleep.
Interestingly, despite the disruption in leptin and in daily eating patterns, Ramadan rarely causes significant changes in body weight.
Investigating why this is the case could yield useful insights into human energy metabolism, said Tom Reilly, a sports scientist at Liverpool John Moores University in England who has studied circadian rhythms and Ramadan.