In the normal course of a mattress' useful lifespan, you may notice the occurrence of slight impressions in the cushioning materials. This is a natural nesting or "conforming" that occurs as the cushioning materials adjust to the shape of the sleeper(s), known as a “body impression”.
Unless a body impression is greater than 1 ½”, it is not a structural defect covered by a warranty. It is simply an indication that the upholstery layers are settling and contouring to your body as you use your mattress set.
Regularly rotating your mattress will help to minimize the depth of the impression that occurs.