It takes some time to overcome this problem. It's cool to learn this, because I can sleep sitting up on the bus(I'm so uninteresting). The way I overcame this was by getting by using the swallow method then I slowly let go of the force until it has become a light sensation on my palate.

Edit: I'm sorry this was directed more to sleeping upright. When I started sleeping on my back I used a pillow by putting it on my chest acting as a support for my mandible.

Sleeping on my side with a pillow completely resolved this for me.  Absolutely worth a try.

Posted by: drunkwithcoffee

Sleeping on my side with a pillow completely resolved this for me.  Absolutely worth a try.

Do you not find the tongue tends to fall to one side of the mouth due to gravity?

Check your nose, it's very likely its getting congested at night and forcing you to breath through your mouth.

@abdulrahman What is there to do about that?

Posted by: Pame

@abdulrahman What is there to do about that?

Allot. A warm saline wash of the nose before going to sleep can help. Combining that with nasal sprays can do wonders. In more sever cases somnoplasty and septoplasty can give long term relief. And of course avoiding allergens such as smoke, dust, animal dander, and pollution is a must for any of the above to work.

Posted by: paradise

Posted by: drunkwithcoffee

Sleeping on my side with a pillow completely resolved this for me.  Absolutely worth a try.

Do you not find the tongue tends to fall to one side of the mouth due to gravity?

Ideally it rests up against your palate I think.

I haven't perfected sleep mewing at all, but I do know that side sleeping/pillow has allowed me to wake up with lips sealed, which I have never achieved consistently in all my life until I started using a pillow recently (for the past few years I've slept without a pillow on my back).  It has also significantly lessened the severity of my sleep apnea.

Take allergy medicine before you go to sleep