Not only what we eat, but also when we eat seems to be of importance for well-being, nutritional status, and health. The regulation of food intake (amounts) operates through feed backs from the periphery reporting to the central nervous system about the energy content of the body. Timing of eating is controlled by circadian rhythms in activity and sleep, internal rhythms being entrained by the external light-dark rhythm. Disturbed behavioural rhythms, e.g. shift work and travelling across time zones, interact strongly with internal physiology. Life-style in the 24-hour society makes people stay awake, eat and sleep at the wrong times with respect to human circadian rhythms in metabolism and performance.