Ditto what Jason said... If the diaper isn't thick enough, you will get leaks in bed.
Try this - Take your next wet diaper, and try to squeeze urine out of it by hand, over the toilet or in the tub. If you can get anything out of it, it is not thick enough. When my bedwetting became frequent enough that I had to start wearing diapers again, I experimented with several different kinds and brands. Some of the companies that sell cloth diapers have daytime and nighttime versions. I figured the "Nighttime" thickness diaper would be all I needed. Yet I still had leaks on the nights that I wet more.
I started using the cloth inserts or "booster pads", and that solved the problem. Some times I only use 1 insert in my diaper, other nights I use 2 inserts.
If the movement of your body during sleep, is enough to "press out" urine from your diaper then you WILL get leaks around your legs and waist. Other than using duct tape to tape your plastic pants to your body, increasing the diaper thickness is the best option.