I have not tried plopping overnight because I feel like having anything on my head PLUS the weight of wet hair would be so uncomfortable to sleep with. I toss and turn alot so I'm sure I would end up pulling the tshirt off my head at some point and totally ruin the curls. Not to mention, end up having to sleep on a wet pillow!
I wash my hair every morning then plop for about 15-20 while I get dressed and do my make up then I diffuse until its about 90% dry. I use a rectangular microfiber towel I got at Marshalls or one of my husbands big tshirts.
I believe waterlily716 has a great video on youtube on how she plops her hair!