Well, here is an article that is some food for thought. While I would suspect the individual who only needs 6 hours of interrupted sleep is VERY rare, the article is interesting in that it brings up the idea that the 8 hour single stretch of undisturbed sleep by oneself or with one partner seems to be a very modern Western thing. Near the end there is some discussion of "segmented sleep". I was going to poke around the internet for some more references to this but don't have time right at the moment
edited to add: I personally find that I can still function when my sleep has been interrupted a lot, but I have to stay in bed until I get a cumulative number of hours that is acceptable...total sleep for me is more important than long stretches. Although boy does a long stretch feel good :) Now, if a mom only has a narrow window to get all the sleep (the 8 hours at night, which is what most people have time for) I can totally see that the interruptions would be really disruptive. I've read Jay Gordon's method for night weaning and although I haven't tried it yet, when I'm ready to nightwean my baby (he's 15 months now, too) I plan to give it a try.
I've already read that article. In fact, I've given it to some of my patients to read to reassure them that a particular sleep pattern that they might have is ok. However, it does not support FIBJ's assertion that needing 8 hours a night is a myth.