The fact that vampires would ever need sleep hasn't really been questioned - they're dead, right? Presumably it's so that they can escape the sunlight during the day and become well rested for hunting during the night. The fact that vampires are nocturnal is what makes vampires so terrifying because they prey upon their victims at a time when most people are most vulnerable - when they're tired or alone in bed. So why the coffins? It is thought that vampires have to return to their resting place in order to well, rest. But the idea of coffins came about because whenever someone was suspected of being a vampire, their graves were dug up. Sometimes their body would be sitting up or would be found in a different position because their body was decomposing. Obviously when you believe in superstition, you do not turn to science for answers and you'll believe that they're not lying down because they will have moved position when they got back into their coffins after a night of terrorising peasants.
