The Science Behind Failed Resolutions - WSJ.com
This helps explain why, after a long day at the office, we're more likely to indulge in a pint of ice cream, or eat one too many slices of leftover pizza. (In fact, one study by researchers at the University of Michigan found that just walking down a crowded city street was enough to reduce measures of self-control, as all the stimuli stressed out the cortex.) A tired brain, preoccupied with its problems, is going to struggle to resist what it wants, even when what it wants isn't what we need.
via online.wsj.com
This explains it! ;)