I've posted on this topic several times about various sleep issues my DS has had (there have been many, his sleep has never been good), but this has to be the weirdest. He's 6mo and has been having perfectly alternate good and bad nights for over a week now - the good nights are the best he's ever had (8pm to bed, df at 10.30 and just one wake-up around 4am for a feed then back down until 7am) and the bad nights are awful - last night he woke up howling just before his df at 10.30pm and it took us an hour to get him back down, then he woke up every hour and a half until he got up for the day at 5.30am! We'd been down to just one feed at nights for some time but last night I fed him loads just to try and get some peace.
Why on earth would this be happening? Do babies have a 48 hour cycle? I've noticed on the bad nights that he sleeps poorly in the day too - not sure if this is cause of effect though.
In a way the good nights almost make it worse - they give you hope only to have it taken away!