Are you a backpack wearer?
One of the things that can trigger a stiffed neck is the bag, specifically, the heavy backpack. Some people are fond of putting items inside the backpacks that weigh like hollow blocks and little did they know that this can cause stiff neck.
What to do then? Stop putting laptops, medical journals that are 2000 pages long and not to mention, the totally heavy hard bound copies of your books and a lot more. Just keep it light to avoid a stiffed neck.