Hours, that is. Every few years a new sleep study gets into the news and we're breathlessly informed about what we really need. Last time I looked at this segmented sleep was the topic, but there were plenty of "You need X hours" reports before that.
I think we all know that the total amount of sleep people need varies with age, and some people get by with dramatically less. The question isn't really "What's the mean of the distribution for my cohort?" but "What do I need?" And, of course, "When?" Siesta, anyone?
Camping is instructive. Unless there's some kind of get-together, I don't stay awake for long after the fire goes out, and I tend to wake up pretty early; at least relative to when you wake up for a 9 to 5 job.
My ancestors came from Europe, where they'd lived for thousands of years, with remarkably few electric lights. I'd think that would be long enough to get adjusted to the fact that nighttime isn't of fixed duration. Sometimes night is 8 hours, sometimes 16--or even worse. Segmented sleep aside, it isn't very plausible that people would sleep the same amount all through the year.
So why would there be an ideal amount of time for a block of sleep at night?
Phooey. I'm going to bed.