Your pillow should support your head so that your spine is horizontal. The best way to test this is to have someone attach some 15mm (approx) wide masking or electrical tape to your spine at the base of your skull and run it all the way down to the top of your buttocks along your spine. Get in bed and lie on you side and have the person check to see if the tape is straight. If the tape dips or arches, you have a mattress problem, but if it dips or jumps up at the cervical spine, you have a pillow problem. The tape should be straight along its length.