h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

You can have too much of them. Four or five pillows on your bed may work great when you're sitting up and reading, but once you're ready to doze off you should probably throw all but one or two on the floor. Being more or less perfectly horizontal is condusive to many physiological changes associated with oncoming sleep, and they simply are harder to come by if you are sitting at an angle rather than flat on your back. An added bonus: you are less likely to wake up with a stiff neck in the morning, because you did not slip down a downy slope during the night and forced your neck to bear the brunt of the angle.