When sleeping in a bed in the recent 1.8 snapshots (I am not sure if it was before 14w07a, but that was when I noticed it), You float 2-3 blocks above the bed, and will glitch into and through any blocks above you, giving you an x-ray view. You do not take any suffocation damage from being in blocks. I am unaware if this is a visual glitch for the sleeper, or if your character is actually floating, as I have no means of using multiplayer to check. If there is already a ticket for this, I apologize, as I was unable to find such a ticket that described what was happening.