it's no secret that we are co-sleepers around here...and sleeping through the night is very hit and miss. during our vacation, c slept through all but 2 nights (a record) but i think it was out of sheer exhaustion from all the time outdoors.
anyhow, a really interesting article...
http://www.kathydettwyler.org/detsleepthrough.html
and a few of the highlights from the article...
"Human children are designed (whether you believe by millions of years of evolution, or by God, it doesn't matter) -- to nurse *very* frequently, based on the composition of the milk of the species, the fact that all higher primates (Primates are the zoological Order to which humans belong, higher primates include monkeys and apes) keep their offspring in the mother's arms or on her back for several years, the size of the young child's stomach, the rapidity with which breast milk is digested, the need for an almost constant source of nutrients to grow that huge brain (in humans, especially), and so on."
"Human children are designed to be sleeping with their parents. The sense of touch is the most important sense to primates, along with sight. Young primates are carried on their mother's body and sleep with her for years after birth, often until well after weaning. The expected pattern is for mother and child to sleep together, and for child to be able to nurse whenever they want during the night. Normal, healthy, breastfed and co-sleeping children do not sleep "through the night" (say 7-9 hours at a stretch) until they are 3-4 years old, and no longer need night nursing. I repeat -- this is NORMAL and HEALTHY."