I camped this past weekend in mid 30 degrees (old 1986 4-season tent).  I do have a decent insulated pad and 15 degree bag, but I find the best way I keep warm is to wear as little as possible inside my bag.  I slept very warm in a pair of shorts and a tank top.  I had to keep unzipping the bag on the second night because I was overheating.  I camped in January in lower temps and felt a little cold, but I slept in wool leggings and a wool long sleeved top.  I was cold when I went to bed so I ignored my own advice.  😄  I do still wear socks, but put on dry socks at bedtime.