Body & HealthFact
Yawning doesn't add oxygen. Here's why you still do it.
The textbook story is refuted: breathing air richer in oxygen does not reduce how often you yawn. Today's leading hypothesis is cooling: a yawn pulls in cool air and briefly boosts blood flow in the head. People yawn more when their brain is warmer, and less with a cold pack on their forehead.
Useful: If you keep yawning while studying, the room may be too warm rather than you being lazy.
Source: en.wikipedia.org