Most modern day mattresses have very generous filling layers for your comfort. Do not worry if hollows appear in the sleeping areas of your mattress, this is quite normal
Your mattress is going through the natural settlement and bedding in stage. It is caused as a result of the fillings compacting under body weight. It is not ‘sagging’ springs etc. Gradually, all the fillings will compress. Regular mattress turning and/or rotating aids this process.
It is very common to notice a ridge in the middle of a mattress. This occurs most in king size and super king size mattresses. This is due to the fact that it does not get slept on regularly. You can reduce this by using the centre of the mattress when you can, to help the comfort layers settle.