During yawning, you breathe in deeply while you hold your facial muscles tightly contracted. Normally, you breathe 15 to 20 times a minute. When you feel tired, you may breathe only half as often, and may not get as much oxygen as you need, which causes nerve cells in your brainstem to make you breathe deeper and faster, and your facial muscles to contract in a yawn.
Breathing deeper and faster brings in more oxygen which refreshes you for a few seconds, but it also causes you to blow off more carbon dioxide from your bloodstream, which can make you feel dizzy and more tired. So yawing also can help put you to sleep, which is the most effective treatment for tiredness. After you nap, you can wake up feeling refreshed.