When someone is exhausted they are more likely to make unwise choices which would end up being quite costly. In fact not only would you be unwilling to spend more time at a store trying to find the cheapest items to buy, you might be tempted to buy anything you can grab fast, find out that you didn't buy everything you need when you get home and probably drive back to the nearest store . . . Go shopping only when you feel energized because you are more likely to focus on the task at hand. You'll make wise choices and save some money.