Being tired can make a person cranky. It can make it hard for someone to see the good in anything or anyone. It can make them want to yell at old people and kick puppies.
Sometimes, all that needs to happen to make a day better is to take a break and take a nap. Find a quiet place and relax. Ten minutes can make the difference between a career-ending meltdown and world peace (at least for *your* world).
So, if you are feeling that you are losing control of your good day, go to your room (or your car, or shut the door of your office) and catch some Zzzz.