The expensive brain hypothesis predicts that the lowest stable level of steady energy input acts as a strong constraint on a species’ brain size, and thus that periodic troughs in net energy intake should select for reduced brain size relative to body mass. Here, we test this prediction for the extreme case of hibernation. Hibernators drastically reduce food intake for up to several months, and are therefore expected to have smaller relative brain sizes than non‐hibernating species. Using a comparative phylogenetic approach on brain size estimates of 1104 mammalian species, and controlling for possible confounding variables, we indeed found that the presence of hibernation in mammals is correlated with decreased relative brain size. This result adds to recent comparative work across mammals and amphibians supporting the idea that environmental seasonality (where in extremis hibernation is necessary for survival) imposes an energetic challenge, and thus acts as an evolutionary constraint on relative brain size.