I work in IT support, which is basically the next pond over from development. Because the job is so mentally intensive, if I'm working on complex tasks for more than 4-5 hours, my brain is catatonic by the time I hit the end of my shift.

Mental effort, is still effort. Most of the time you can't see that someone is mentally tired, but it is just as debilitating as being physically exhausted.

I can not do my job while mentally exhausted. One screw up from me, and I have the ability to, entirely by accident, take out an entire organizations ability to do useful work.

Some of my clients, I've seen log into the system at 8AM or earlier, and still be online after midnight. I don't understand how they're getting anything useful done by that time.