Oh I feel for you - we had to do this. My LO used to sleep right through from 9pm to 8:30am from a very early age. Unfortunately we moved house when she was 16 weeks old and it all went to pot.
Initially we thought it was because we'd moved, so I put up with the renewed waking up all night. We kept her bedtime routine exactly the same but she still woke up. It was so sporadic as well that it was hard to rely on. Sometimes she awaken every half an hour, sometimes she'd stretch to as much as 2 hours of sleep. After a while, I was so tired with the broken sleep that we needed to do something.
She was approaching 6 months and she wasn't hungry as when I picked her up, she fed for all of 2 - 3 minutes and then was asleep again. Plus she had been sleeping through before we moved. We even put her on solids to see if that would work. It didn't.
By now my partner was sleeping in the spare room just to get some sleep for work!
We eventually tried the controlled checking technique. I'd already started it a couple of weeks earlier for the daytime naps and it went really well, so I hoped that night time would be easier.
Like the PP, I'd put her down awake and then after 1 minute, go back, pat her back and say "sshhh, sleep time" and then go again. After 2 minutes the same thing, then 3 minutes, then 4 minutes, then 5, then 6 etc. I was told to never leave a baby this young crying for more than 10 minutes, so after checking on her at 10 minutes, I just kept it around that time and checked every 10 minutes. I'll add here that I was still in the room (which I realise now may not have helped, if she could smell the milk on me), she just couldn't see me.
The first night it took 1 hr and 20 minutes to settle her but she slept until 4am!! I was so tired that I didn't think and picked her up to feed her at that point, where she fed for just a tiny time and then asleep again until 8.30. Bliss!
The 2nd night, it took 20 minutes to settle her and while she woke up at 3:30am, she simply grizzled and after afew seconds, she put herself back to sleep.
The 3rd night she settled without me needing to go back - I think we had about 15 seconds of grizzle and then she was asleep and she slept through until 8:30am.
So first night was really hard, but this technique worked for us really well. There are other techniques though and it's whatever you feel comfortable using.
