Dear S.,
Sorry, I have no advice - just my story and opinion (I seem to say that a lot here . . .)
I think that this is a personal thing - meaning everyone is different and will sleep through the night on their own schedule - some say that bottle fed or cereal fed babies sleep better longer - my own situation refutes this. Everything I read at this time said that sleeping through the night for an infant is different than how we normally think of it - that sleeping through the night for babies means 4-5 hours at a time. My son (bless his heart), at five weeks (and by the way, breast-fed only) went five hours one night, then ten hours the next. The third night, he woke up after five hours again, but the fourth, he slept ten hours AGAIN. On the fifth night, when he woke up after five hours, I said, "No way!," stuck his pacifier in his mouth and patted his bottom and he went back to sleep for another five hours. He slept ten hours through the night from that point on . . . My "baby" was also a very good sleeper (and also breast-fed only - at least at this point) and was sleeping through the night by the time I went back to work when she was six weeks old. (This was always a concern for me because I have worked graveyard since my son was born, and worried about asking someone else to get up with my baby during the night.) We went through some other issues with my baby, however, and at six months, I was told that I needed to wake her and feed her during the night (which I found slightly insane). I was able to stop feeding her through the night around 12-18 months. But when she was three and four, she was still not able to get through the night without getting up. It was my mother who made the connection between her still getting up at this age, and the fact that I had been instructed to wake her and feed her through the night as a baby. (The good news is that she did eventually out grow that habit - thank God! - and at fifteen sleeps through the night just fine.) Circumstances when my middle child joined our family were not easy from the beginning - though I had a normal pregnancy and delivery (though REALLY fast!), and even though she was my biggest baby at eight and a half pounds, she spent a week in the NICU with pneumonia, my two year old had a horrible stomach flu a month later that lasted an entire week, and then one week after returning to work, I came down with chicken pox (which of course I passed on to the two year old and then the two month old! It was the longest six weeks of my life!) So, wouldn't it just figure that her sleep pattern would be screwed up, too. I don't know when it really started, but I noticed it when I was nearly hysterical with fever from the chicken pox. She wanted to play all night long. She was wide awake and happy - as long as she had company. But I had a fever - and it was going up. I finally told her, "I don't care what you do, I need some sleep, I have to get up with your brother in a couple hours," and I parked her in her bassinet across the house (and then listened to her cry through the vents). I tried to keep her awake during the day so that she would sleep at night, but when I was too sick to watch the kids and gave them up to my husband, he simply followed her lead, so when I was better, she was up all night and sleeping all day. Now this I fought - I woke her up every two hours during the day to feed her in an attempt to get her on a more normal day/night sleep schedule. Honestly, if she had been my first baby, I don't know if I would have ever had another. (And by the way - she is still my most difficult child at seventeen!)
I know that we all need our babies to sleep through the night, because our lives don't stop when we are blessed with their presence, but unfortunately they all do this in their own time.
Good luck! And remember, this too shall pass!
B.