For years the brain has been compared to muscles (although it is not composed of muscle tissue). Muscles, when overexercised, become depleted. In that sense the brain is like a muscle. When depeleted, the brain becomes less effective. It can be important to understand this if you have a major decision to make. Try to make that major decision with a fresh brain. If you've just made a series of minor decisions, or needed to exercise self-control about something, your brain may be somewhat depleted and the choice you make about the major decision may be less than desirable.