TOO SOFT MATTRESS IS BAD FOR NECK.
How too soft a mattress could hurt your neck.
We may have heard about back pain resulting from sleeping on too soft a mattress but not many people know that it can also be bad for your neck. Soft mattress will relatively make your pillow higher than it should because your whole torso sink in the mattress more while your head remains somewhat the same. This increase the angle of your neck to the spine. Try using softer pillow if you feel that your mattress is soft but still supportive. But if your mattress is too soft, it is always better to fix that by getting a firmer and get a matching pillow to your body size and sleeping posture.