what about the feelings of depression and discouragement we get when stuck at boring iobs or with problems we cannot solve? Such feelings resemble those that accompany physical fatigue, but they do not signify genuine depletions because they often easily t.rporrd to changes of context, interest, and schedule. Nevertheless, the similarity would be no accident, for proU- ably those feelings arise because our higherlevel brain centers have evolved connections that exploit our ancient fuel-exhaustion warning systems. After all, the unproductive use of time is virtually equivalent to wasting hard-earned energy