Sleeping on a sofa occasionally will not do it any harm. Sleeping on it on a permanent or long-term basis can result in wear and tear, and you may find that eventually some parts are affected more than others. It depends on the height and weight of the sleeper and the position in which he sleeps, as well as whether bed linen is used between the sofa and the sleeper. The upholstery of the sofa may also sustain damage, and fabric-covered sofas may require frequent cleaning which will reduce the lifespan of the furniture.
The seat cushions are likely to sag in the places where the body weight is greatest on a regular basis. In a sleeping position, the greatest weight is around the hips and middle body area, and this is likely to cause the center seat to sag faster than the sides. This can also be a result of inferior cushion material that breaks down with use.
Areas where pressure is placed on the foam padding of the upholstery on a regular basis may eventually remain depressed, unless it has a high "recovery factor," which is a reading based on the indentation force deflection standards used by the foam industry. These areas could include the arm rest of the sofa, where the sleeper puts his head and possibly his feet, and the lower back rests, which are not intended for the partial weight of a sleeping body.
The sleeper should always use linen or a blanket to place between his body and the sofa, as regular contact with the cover of the sofa will result in scuffing and wearing of the cover material. If the sofa is covered in leather, the surface may start to show signs of dryness and cracking, which will lead to tears in the leather. On a fabric-covered sofa, the catching of a toenail can tear it much earlier, and the position of the feet may mark the cloth. If the sleeper uses body lotion or hair products, these may leave stains on the upholstery, even if it is micro-fiber. Regular or frequent steam cleaning may help to keep the sofa looking good, but in the long term this will lessen the life-span of the furniture.
Many people toss and turn in their sleep, and regular movement of a body on the sofa could result in damage to the framework. The sofa is designed for people to sit down on it consciously, and unconscious movement can be far more violent. A broken support will require opening the upholstery to repair it, and this may require the attention of a professional upholsterer.