What would cause the feeling of vibrations in the head and/or back particularly when lying down?

Vibrations: The sensation of vibrations can stem from minor tremors of muscles in your neck or back. Another possibility is mild irritability of superficial nerves in those areas. Apparently you're not aware of these sensations when you're up and about or active. I doubt there is a serious problem but if you're still concerned, consult your physician or a neurologist.