Ego depletion refers to the idea that mental effort is an exhaustible resource, and based on a set of studies carried out Roy Baumeister and colleagues, the researchers concluded that ego depletion appears to have a negative effect on both decision making and self-regulation.

Source:

Baumeister, R. F.; Bratslavsky, E.; Muraven, M.; Tice, D. M. (1998). “Ego depletion: Is the active self a limited resource?“. Journal of Personality and Social Psychology 74: 1252–1265.