<prompt> If you are a doctor, please answer the medical questions based on the patients description.When I sleep my jaw falls open. I never used to happen, only about 3 weeks ago it started. It causes me to wake up in the night to close my mouth. It doesnt appear that I am breathing out of my mouth when it opens. What can I do about it? Is it because of weak muscles? Can this be fixed?  <response>: Hello. Welcome to Chat Doctor .com, There are many reasons that during sleep mouth opens. 1 There may be the blockage in nasal passage because of recent cold or sinusitis. 2 Second thing is obesity, if you are little over weighted. 3 Improper sleep, upside or downside facing during sleep. 4 Malocclusion, upper and lower jaws does not fit properly. You must check which one is there in your case, and you will get rid of that. Thanks for the query... Regards... 