I remember reading somewhere that deva's are against fainting when exhausted. What I'd like to propose is screen fade-out. Think it was in the 18 wheels of steel. When character was tired the screen was fading out for something like a second every 5s(?). If you were trying to cope with it, at later stage it was getting worse, up to a stage where fadout was getting longer than an actual display. I think it's a good compromise between enforcing some better RP while still being better than just collapsing from exhaustion.