http://www.minecraftwiki.net/images/thumb/8/89/Bed_Template.png/200px-Bed_Template.png
Kilder

In earlier versions, sleeping in a bed placed in an area that is exposed to monsters would cause the player to wake up with a zombie or skeleton next to them. As the chart to the right shows, placing beds next to external walls would also cause this.
As of release, players will not be attacked when trying to sleep anymore. However, if mobs are within 10 blocks of the bed when the player tries to sleep, it won't let them.