Waking up with a stiff jaw in the morning may be due to cold, improper posture, temporomandibular joint disorder syndrome and other factors. 1. Cold: When you don’t pay attention to keep warm while sleeping, resulting in localized cold, the phenomenon of waking up with a stiff jaw in the morning may occur. 2. Improper posture: When sleeping, improper posture for a long time will lead to localized compression and blood circulation obstruction, which will lead to the phenomenon of waking up with a stiff jaw in the morning. 3. Temporomandibular Joint Disorder Syndrome (TMJDS): usually due to trauma, mental stress, night grinding and other factors that lead to lesions in the joint area, resulting in pain, stiffness and other uncomfortable symptoms. When you wake up in the morning with a stiff jaw, you can’t tell the exact cause just by one symptom, but you need to combine it with the relevant examination and clinical symptoms to make a judgment.