In addition to the answers given, re. age and other variations, I have heard that army training can work on batches of 3-hour sleep periods. Apparently that is meant to be the optimum minimum amount, allowing enough rest to be useful.
I imagine that this would jeopardise health, though, in the long term. I think I am in danger on that front in writing so late (early in morning), so apologies if this is not as clear as I meant it to be!