Minecraft wiki entry for Bed, associated text: For a location to be unobstructed, the block at the level of the bed must be air or non-solid (e.g. torches, but not glass) and there must be a space with a solid block below it and two non-colliding blocks for the player to stand in 0-2 blocks below the bed. It does not matter if the bed itself has blocks above it. Putting a slab one block above a bed can act as a two block tall space, as the bed is half a block tall. The bed never spawns the player on or directly below itself even if all other locations are obstructed. If a bed is obstructed, the player's spawn point is cleared after they respawn. That is, even if the bed is subsequently made usable again, the player continues to respawn at the world spawn until interacting with the bed again.