Circadian rhythms, which are influenced by the light and dark that an organism is exposed to, have important effects on physiology. Traditionally, laboratory studies of circadian rhythms have used a 12-h light and 12-h dark experimental set up (photoperiod). However, apart from regions close to the Equator, the photoperiod varies throughout the year. A study in Cell Metabolism has now investigated the effects of different photoperiods in mice.