Scientists have identified a cellular, thermodynamic mechanism that explains the switch between night and day activity in mammals.[1] Using diurnal thermal or osmolar cycles, they found opposite entrainment of diurnal versus nocturnal cells, reflecting the temporal niche of their species.[1] Mouse and human cells differed not only in size but also in the direction of their response to temperature, with temperature change inducing opposite shifts in global protein synthesis and phosphorylation.[1] These differences involved differential sensitivity to the mTOR and WNK kinase signaling pathways.[1] Comparative genomics has confirmed the accelerated evolution of genes in these pathways in diurnal mammals, which increases the resistance of their cells to thermodynamic perturbations.[1] Inhibition of mTOR in nocturnal mouse cells, tissues, and in vivo shifted circadian timing toward diurnal activity.[1] These findings link cellular responses to thermodynamic changes to circadian entrainment through genetic changes in the mTOR and WNK pathways.[1]