What happens if your mattress is too soft or too firm?

On a mattress that's too soft, you sink in too deep and your spine stays under strain all night, which can cause discomfort in your lower back. On a mattress that's too firm, your body doesn't sink in enough and has to adapt to the mattress in an unnatural way, which can put strain on your neck and lower back. Neither extreme is ideal. The right firmness matters more than "as firm as possible". Experiencing persistent discomfort? Be sure to consult a specialist.