How yawning works
When you yawn, you open your mouth wide and take a deep breath. This action stretches the muscles in your jaw, face, and neck. At the same time, your body increases blood flow and oxygen delivery to your brain. These physical changes can help increase alertness and prepare your body to be more awake.
Why it happens when tired
Your brain needs oxygen to stay alert and focused. When you become tired, your brain's oxygen levels may drop slightly. Yawning is your body's natural response to boost oxygen and blood flow, essentially trying to keep your brain active. This is why you often yawn right before bed or when staying awake becomes difficult.
Temperature regulation
Recent research suggests yawning may also help control brain temperature. Your brain works best within a specific temperature range. When your body temperature drops or your brain gets too warm, yawning might help adjust it back to the ideal level. This could explain why you yawn more in certain seasons or times of day.
Other times you yawn
You do not only yawn when tired. People also yawn when bored, stressed, or anxious. Yawning can happen when you see someone else yawn, which is called contagious yawning. Even babies yawn before they are born, suggesting it is an automatic body function rather than something learned.
What scientists still don't know
Despite studying yawning for years, scientists have not found one single complete explanation for why we yawn. The exact triggers remain unclear, and different reasons might cause yawning in different situations. Researchers continue to investigate this common but still mysterious behavior. 
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