It's not just less sleep. In my personal case (I know, anecdote blah blah) I can be very productive, happy and more functional if I can consistently sleep for

It's not just less sleep. In my personal case (I know, anecdote blah blah) I can be very productive, happy and more functional if I can consistently sleep for 6-6.5h per night as long as my wake up time is no earlier than 9:30-10:00. I don't even need caffeine at all, and I can still be perfectly fine with the occasional 5-5.5h sleep night. OTOH, I might have slept 9h every night for days but if I have to be up earlier I will be miserable, my brain will be foggy, I'll have less energy and I tend to injure myself a lot more working out.