Meal timing and composition are frequently reported in the literature as zeitgebers (that is, time cues) for the circadian system of humans and animal models, albeit secondary to light. Although widely assumed to be true, evidence for food zeitgeber effects specific to humans is notably scarce. Fostering zeitgeber hygiene in the general population as the development and practice of healthy use of zeitgebers could potentially reduce chronobiological strain, which is defined as disruption or misalignment within the circadian system. Such chronobiological strain is associated with modern 24/7 lifestyles (for example, shift work) and several negative health outcomes. Adjustments to meal timing and composition are an attractive strategy to synchronize circadian rhythms and develop zeitgeber hygiene. Thus, clarifying the actual effect of meal timing and composition on the human circadian system is a crucial piece of the human chronobiology puzzle. This Review weighs the evidence from human studies pertaining to the hypothesis that food is a circadian zeitgeber by comparing findings against formal zeitgeber criteria put forward by Jürgen Aschoff in the 1950s.