Here’s a concept for you: depletion theory. According to the behavioral economist Dan Ariely, our ability to make any type of difficult decision is adversely affected by fatigue. And self-control is an exhaustible resource. This is why by the end of the day or the close of a meeting, it’s harder to concentrate. Or why it’s harder to turn down that Snickers bar that would ruin your diet at 5 pm than it was at 3 pm.