It's colostrum, it's what it made before the actual milk comes which will not come until about 5 days after the baby is born and as for leaking only at night that's mainly when I leak and I'm guessing it's because if you lay on your side it's putting pressure on your boob and it just leaks? I started leaking around 17-18 weeks... Just wear breast pads to bed so it doesn't get on your sheets or anything