I don't think it's a good idea to restraing your slave during sleep on a regular basis. Anyone moves uncounciously during sleep. If you restrain it the slave will necesarly wake up numerous times, which is why I don't do it. I prefer my slave to be in shape in the morning.

Mine always sleeps tied (very loose) to the bed frame, most of the time with only one leg tied. I use a leather restraint with a leather which goes through the frame and comes back again. It has a ring at the end which is locked with a padlock to the restraint. It's about two feet long. I already used the strip for suspension, so it's very very solid. I tried a chain in the past but it makes too much noise during the night.

Sometimes she wakes me up so that she can get permission to get to the bathroom, which I always grant and unlock her. However, it gets me grumpy which I find to be great way to start a sincere, truly-deserved punishment, sometimes right away but most of the time the next morning.