What is an adjustable bed? It is a stationary bed that can be adjusted to different sleeping positions. You can adjust your upper body separate from your lower body. The mattresses for these types of beds must be very adjustable and flexible. I really do not think it matters if the mattress is thin or thick. But if it is thick it will protect your body from feeling the springs of the bed or other rough spots that may be a part of the construction of the bed. The mattresses will range in price from cheap to reasonable and you will find some very expensive foam mattresses for adjustable beds. The mattresses come in all the standard bed sizes.