I might throw a spanner in the cogwheels...
A lot of posts go along the way of "monks are contemplative and have very little physical exertion, hence they don't have to sleep as much as we do".
I have to disagree in 2 ways:
1 - I spent some time in a monastery, and there was a lot of work to do. Monks and lay residents spend easily 4-5 hours every day working physically (cooking - lay people only, washing up, housekeeping, gardening, repairs, etc.)
2 - I have a brother-in-law in the French army (Legion Etrangere). They have been told that scientific studies show that the human body does not need more that 3 hours of sleep per period of 24h. In practice, they are never allowed more than 5 hours sleep per day, and their days are filled up with physical training.
A few years ago, I bought for myself a method to reduce your sleep time to 5 hours. It worked for many people, but I was just too lazy to actually implement it for myself.
In short: the sleep time required by the human body is probably over-estimated.