Time as a cue for human hunger

Many people eat at around the same times each day. This may result in meal times becoming predictive of eating's potential to be rewarding at that time (i.e., hunger). Time as a predictor of food reward has received much attention in animals but less in humans, even though many people organise eating around breakfast, lunch and dinner - words implying particular times of day. In this narrative review we examine if and how habitual meal times might cause hunger in people. We start by addressing a basic premise, can time-of-day predict food availability outside of modern clock-driven societies? We find it can, with moderate-to-strong evidence. We then turn to a harder question, is there evidence that habitual meal times can cause hunger? In animals, this focusses on the behavioral and physiological correlates of hunger, which provide positive weak-to-moderate and strong support, respectively. In humans, this means finding studies that control a key confound, namely people often eat after a period without food. This can generate other cues (e.g., an empty stomach) that may cause hunger, potentially resulting in cue competition. Currently, the human evidence is supportive, but weak. Finally, we evaluate models of how habitual meal times might cause hunger in animals, and then critically apply these to humans - noting that several key questions remain (i.e., cue competition). The implications for the human cephalic phase literature, the health impacts of regular/irregular eating patterns, how time may develop as a hunger cue and cause hunger, and its neural basis, are discussed.