Abstract

The human biological clock, which regulates processes from gene expression to behaviour, like that of most organisms, synchronises to the Earth's 24 hour rotation using signals from the environment (zeitgebers). This synchronisation is an active process called entrainment [1]. Daily rhythms also persist in temporal isolation [2], deviating modestly from 24 hours (hence circadian). Light is the strongest zeitgeber for all circadian systems, yet social cues also entrain human clocks [3], possibly via behaviour, for example changing light exposure by closing eyes during sleep [4,5]. Here, we ask what zeitgebers entrain the human clock in real life by exploiting the common discrepancy between social time and sun time. Our results show that the human clock entrains to sun time.