A study in the UK has found changes in cells that explain why mammals, including human ancestors, changed from nocturnal to diurnal, providing clues on one of the most relevant events in mammalian evolution.  

All early mammals slept during the day, while larger predators were active, but several lineages evolved to be diurnal after the extinction of dinosaurs. 

Researchers from the MRC Laboratory of Molecular Biology, UK, studied how cells and tissues from different diurnal and nocturnal mammals, including humans and mice, coped with acute, long-term, and cyclical stimuli that vary during the day/night cycle (e.g., temperature). By monitoring global protein changes, as well as genes that act as reporters of the body’s internal clock, the researchers found opposite patterns in diurnal and nocturnal animals.  

By comparing the genomes of different species of mammals, they discovered genes that evolved together with diurnal preference and so were promising candidates for causing the nocturnal/diurnal shift. By blocking those genes using drugs in cells andtissues, they discovered that two major regulators of cell signalling – mTOR and WNK– were linked with diurnal preference. Mice that were fed a diet that reduces mTOR activity started to behave more like diurnal animals. 

The observation that external factors, such as temperature and diet, influence animals’ circadian rhythms suggests that climate change may alter the time of day when mammals are active, as these factors are affected by climate. 

The study published in Science also carries implications for circadian medicine, a recent field that studies how the timing of treatments can influence their effectiveness, and for animal research, on how to better incorporate and consider species’ different circadian rhythms.