Why do zombies and skeletons in minecraft wake me up while im sleeping?

I'll go to bed in game and ill get woken up in the middle of the night by zombies and skeletons. I don't know why. I have and underground shelter walls are 10x10 well lit by many torches, door to leave to go outside and doors to get to to caves im working on. Ive tried moving my bed so its not touching the walls (im already underground). Enlighten me on what will fix this. Thanks if you can help.