Elevate your legs up on pillows or something. This is very common with a lot of people, pregnant or not, it's called edema. It happens when your legs aren't moving because your body isn't pumping it near as efficient.
Try keeping the elevated when your sleeping. Have them higher than your hips if possible. That should help. Maybe exercise them before you sleep. Move your ankles around in a circle and up and down. I too have swollen feet but mine are swollen all day and go down when I'm sleeping. But keeping them up and moving them seems to help.