I am breastfeeding all other feeds but started last night topping up the evening feed (around 10 o clock) with formula she took about 60ml but then didnt wake up at all until 6am so was asleep 8 hours. With fully breastfeeding she was waking every two hours. Is this ok? it seems like along time to suddenly sleep - do I need to wake her up earlier? give her less formula? or just go with it?