What position should I sleep in, and is there a ‘right’ way to sleep?

After 50 years of research, eminent Stanford University sleep researcher William Dement reportedly said the only solid explanation he knows for why we sleep is “because we get sleepy”. Even though sleep may be, as one researcher put it, “the only major behaviour in search of a function”, it clearly does matter for our health and wellbeing. But are we doing it right? What does the research say about sleeping position?