Sigmund Freud proffered the two distinct systems of the id and the ego. The research by psychologist Roy Baumeister suggests that the ego drains some well of energy which recharges after rest and reward. Exercising self-control, will-power, prosocial behaviours, and volition can drain this source and subsequent execution of such behaviour will be diminished. It has been shown for example that judges are less likely to grant parole before a break or eating a meal.
Current theories posit that there are psychological signals which control the flow of glucose into the brain which would affect this behaviour but more studies need to be done.