In the first few weeks I would formula feed from 10pm and start breast feeding at around 6am. I don't think it affected my supply, but just to be safe, I started to give a bottle of formula at 10pm and then at his next 2 feeds I would give breast plus a top up of formula.
When I changed to this i started to hate the formula feed as, like other ladies have said, breast feeding is much less effort!
It's all about trial and error, I have found what works for us, but its taken about 9 weeks to get us into a nice routine. The only reason I still give 1 bottle per day is that I don't want Finn to lose his interest in bottle feeding, as I know that when it comes to weaning him off the breast it will be handy if he is already used to taking a bottle!
I was under the assumption that ff babies slept longer, which is initially why I started off ff at night, but it makes no difference to Finn.
Currently, Finn is exclusively breast fed, goes to bed at 7 after breast, and has a bottle at 11 ish. He will then wake every 3 hours.
I definitely believe sleep is down to the baby rather than method of feeding. Finn is an extremely hungry baby, and needs to be fed overnight, if he didn't then he wouldn't wake. He won't feed when he's not hungry so I know he is waking for milk and not for anything else. I actually find that he sleeps longer after his 7pm breast feed (4 hours) than he does after his 11pm ff (usually 2 and a half/3 hours).
Basically, for us, whether he has formula or breast he will still wake within 4 hours. This makes no difference if he takes 6oz of formula or 3oz, he will still want feeding a few hours later. I'm sure he is full after 6oz, so the theory of sleeping through after a full tummy doesn't necessarily work! I so wish it did though, I can only dream of getting more than 3 hours of slee in one go at night! x