Unless you spend quite a bit of money, nightly use of a sofa bed is not a comfortable long term sleeping solution if you value your back.
Consider a day bed (with or without a trundle). A day bed consists of a bed that houses a single size mattress, which is 39" wide by 79ish" long. Typically the framing of a day bed will have a back and two sides (which double as the foot board and headboard of the bed). Because the mattress is wider than your typical sofa, it is wise to use wedge pillows along the back and throw pillows so you can sit comfortably on the sofa bed by day. A trundle mechanism is available to create a larger sleeping area. The trundle is a pull out frame that goes under the daybed, which holds a twin size mattress. The trundle can be pulled out and up to meet the day bed mattress, creating a king size bed in an instant.
If you go the trundle route, you have to be sure you are getting the style where the second mattress can be pulled up to the daybed mattress level. Not all trundle styles lift, but rather, they create another sleeping area for another individual on floor level.
Personally, I would rather have a daybed than a sofa bed if it were my nightly sleeping spot.
See images below: