For centuries people slept (relatively compared to contemporary preference) sat up in bed. This is now thought to be what accounts for the shorter lengths of beds historically, while it was previously assumed that people were just shorter! (this might be complete rubbish, but I seem to remember reading it somewhere)

Sleeping in a reclining, or even upright seated position is practised by sleep and dream yogis. Such a sleeping position leads to higher incidence of isolated sleep paralysis disorder and some have speculated that this custom accounts for many literary and historical 'hauntings' centred around the bed, like in Wuthering Heights in which the conditions of the haunting fit the description of sleep paralysis quite well.