Aside from layering your clothes inside your bag, or layering your bag using a bag liner, it's also important to make sure you have a good ground sheet or sleeping pad that will insulate you against the ground and reflect your body heat back up. You lose a lot of body heat into the ground while sleeping, so at the very least, using a survival blanket as a ground sheet (shiny side up) will reflect some of that heat back up. You also lose a lot of heat through your head, so bring a toque or warm hat to wear while you sleep, this adds a surprising amount of warmth and comfort.
Eating will keep you warmer too. Deer spend a lot of time feeding in the middle of the night when it's cold out, not because they're hungry, but because it's fuel on the fire and keeps them warmer. Eating a good hot meal before bed will help your body generate more heat during the night. Emptying your bladder will also help, your body wastes a lot of energy keeping a full bladder of water warm.
Tricks for warming up in the middle of the night include preparing a thermos of hot soup or hot drink just before bed, and drinking it when you wake up cold. In the himalayas everyone carries a pee-bottle for when they have to pee in the middle of the night. Instead of getting out of their warm bag, they'll pee inside the bottle, then keep the warm pee-bottle in their bag until morning.
Personally, I pack a warmer bag, I sleep with a -10ºC bag in the summer time, if it's warm enough out I'll use it as a down comforter when I lay down at the end of the day, but I always end up zipped up inside it sometime during the night. Even in the middle of summer it still dips down close to freezing at night here in the rockies.