biological clock

Why we are daytime creatures

Like the vast majority of primates, humans are diurnal. We eat, play, work – and usually mate – during the day. At night, we sleep. A group coordinated by biologist Mario Pedrazzoli is now proposing a genetic explanation for how this pattern of behavior appeared. Pedrazzoli and his team at the University of São Paulo (USP) believe that a peculiar genetic trait defines these behavioral patterns, which are associated with 24-hour circadian cycles of hormonal release: tandem repeats of a specific segment of the gene PER3. Previous studies had already shown that PER3 controls the functions of a region of the brain believed to be the control center for mammalian circadian rhythms, responsible for regulating the 24-hour biological clock of metabolism and behavior. In 2005, Pedrazzoli had observed that people with five tandem repeats of this specific stretch of PER3 were at greater risk of developing a sleep disorder than those with four repeats. Now, in their investigation of the evolutionary origin of these repeats, Pedrazzoli and biologist Flávia Cal Sabino verified that they are exclusive to monkeys, apes, and humans (PLOS ONE, September 2014). Out of the 13 primates they studied, the smallest number of repeats – only two – was found in Aotus azarae infulatus, a nocturnal monkey.