I sometimes have to pull off tight deadlines that require me to go three or four days with only a couple hours of sleep each time (it's grad school, it sucks).

Burnout for me feels like the third day of that all the time. A fatigue so deep it's almost an ache. The thought of booting up your IDE feels like the equivalent of sitting through a lecture after that third day of little to no sleep. Sure, you can do it, but it just seems like such a chore.

That's burnout. It takes the things that invigorate you and turns them into chores. Then it breaks both your legs. Have fun getting your chores done.