Physical exertion plays a role - but when I'm rested (~8 hours of sleep) I usually don't get really tired until I've been up 18-20 hours. I've worked with frie

Physical exertion plays a role - but when I'm rested (~8 hours of sleep) I usually don't get really tired until I've been up 18-20 hours. I've worked with friends that are completely different, and struggle to keep their eyes open after midnight. So there's definitely big differences. I do generally manage to function well on "average" of ~8 hours of sleep every 24 hours: some night 8,9 or 10 some 4 or 5. But yeah, if you're one of the people that can't understand how "changing the rhythm" can be a problem: try starting tour day at 0230 or 0300 am for a couple of weeks. For the vast majority, that is though (hence attack right before dawn...).