After tummy tuck surgery, is it normal for your legs to go to sleep, numb, while you are asleep at night?

This may be due to nerve irritation, the compression garment or other causes. Discuss this with your surgeon. 

This would be unusual and not a typical post operative complaint for abdominoplasty.  The surgical field does not involve any significant concerns that would effect low extremity nerves.  However, position for long duration under anesthesia or post operative positioning at home during recuperation could potentially contribute to these symptoms.  Certainly you should follow up with your surgeon, but ambulation and repositioning while will likely enable this to resolve.

This is most likely due to a change in your sleeping position. Unusual pressure/position in particular areas can cause nerve/blood supply compression and be the source of this temporary sensation you are experiencing.