Humans faint due to certain reasons.
1. The heat can tire out the brain, so it ends up shutting down.
2. Stress and anxiety causes the brain to overload, therefore, the person faints.
3. If people cry far too much, they can start hyperventilating, which causes them to stop breathing. The brain stops for a split second in a attempt to get the lungs working again.
That's all I could think of