You have the ability to mentally push through a lot of stuff, but there are a host of signals your body will use to warn you such as: cognitive performance (although this is hard for yourself to actually admit its happening), behavioral indicators like yawning, head bobbing, frequent posture changes, and slowed speech are just a few indicators. Irritability is another big red flag.

I've only had two instances where I ignored all the signs and continue to push on until my body literally shut down on me. Finals week in college as an architecture student trying to finish a final studio project, and study for four finals. 5 days, 3 hours of sleep. That was one of them.