Really misleading headline. The study wasn't about hammocks at all, just a normal bed that sways. Makes me think more of being on a boat. I haven't experienced the effect, but David Foster Wallace wrote about it: http://www.harpers.org/media/pdf/dfw/HarpersMagazine-1996-01...
(Not going to give the page number of the sleep thing because I believe everyone should read the whole thing :)