From the Economist (Sep 18th 2008)
No actually I don't think it does. A couple of Italian patients with some degenerative disease and weird sleep patterns may be interesting for medical research, but studying them doesn't tell us much about the purpose of normal sleep.
Let's try out a much simpler example. We observe that eyebrows deflect sweat from the forehead and help protect the eyes. We might reasonably infer that this effect is the purpose for our having eyebrows at all. Even if a few people don't have eyebrows, and perhaps have developed some other mechanism for keeping sweat from dripping into the eyes, that doesn't force us to change our opinion on the purpose of eyebrows.
There are lots of biological and psychological functions in the human mind-body system. Sometimes these functions are broken or don't work properly. In the works of Oliver Sacks, for example, we can read many anecdotes of patients who have adapted, sometimes in curious or heart-warming ways, to their broken systems. But even if a system doesn't always work properly, that doesn't mean we can't appreciate its proper purpose when it does work properly.
Maybe it is possible to be both awake and asleep at the same time, as researchers now speculate. But that is not challenging our notion of the purpose of sleep, it is challenging our notion of the very nature of sleep. Not the same question at all.