Abstract
A recent elegant study published in this journal (Zerbini, Winnebeck & Merrow, J. Pineal Res, e12723, 2021) reported data on weekly and seasonal changes in circadian timing as assessed by the melatonin rhythm in dim light in a population which was exposed to a change from standard time to day light saving time. The authors highlight a one hour earlier timing of melatonin onset in summer compared to winter and a 20 minute delay on work-free days compared to work days. The variations in the timing of the melatonin rhythm are reported in standard time and the authors imply that the data are consistent with synchronisation to midday and that ‘we know that humans entrain to suntime’. Here we show that their extensive data are most parsimoniously explained by entrainment to local clock time and associated light exposure rather than ‘sun time’, indexed by either dawn or midday.