In 1.12, go to config/rftools/dimensions.cfg. There will be a line under general called bedBehavior. If you set it to 2, it will set your spawn on sleeping instead of disallowing sleep.
You may also want to enable B:respawnRfToolsDimension so that you are not kicked back out when you die. There are a lot of other useful config options in that file too, look around!
However, it doesn't seem like it's possible to actually sleep and set time to day. I think this is due to the existence of frozen-time dimensions and the problems that could cause, but it is a little unusual that it's not enabled for dimensions where time does move. I will edit this post if it does become possible.
If you are living in a dimension, just hole yourself up for the night and pretend beds are just spawn points. Sleep is for the weak.