Yes! 

I started out with the same problem, but I promise you, after like 2 days of mewing I kept my teeth together without even thinknig about it. Its all about the musclememory 

Taping the mouth shut at night helps me a lot. Nexcare Durable Cloth Medical Tape is the one I use. Sticks very well but peels off easily. You can get it cheaply on Amazon.

Also, practice. This is something I'm working on as well. It helps to remember that simply using suction to quickly get your tongue into position is basically all that's needed. I'll flatten my tongue on the roof of my mouth and suck it back quickly and then it's in position. Whenever I feel it's not in this position, repeat, and get it back to position. The more you do it the closer to auto-pilot you'll get.

Good luck!

Posted by: @rezgrio

Hello love so much this forum so far!! my question is while sleeping do you have to keep your teeth together too? I am getting more and more used to keep mewing while sleeping but seems impossible to me to keep teeth together when I am turning off. It is enough with the tongue on the roof? I am keeping my mouth close too with no problem so far but when I am relaxing I can't keep my upper and lower teeth together and just have tongue on the roof and mouth closed. Thank you very much in advance!

Stop being week and repite the behavior