I once had to go a few days without sleeping due to some emergency situations but on average it depends what I'm used to. When I was used to waking up at 6 I was okay waking up at 6. Now that I'm used to waking up later, even if I get the same amount of sleep time, if I wake up at 6 I will feel terrible. I've also noticed I can go to bed at 9 and wake up at 6 and feel worse than if I went to bed at 4 am and woke up at 10 am.