Your milk supply is regulated by how much breastmilk your baby takes from the breast. If the baby is getting formula he will take less breastmilk, which will reduce your supply. Normal infant sleep patterns vary widely, so try to resist the urge to compare your baby with other babies. Parents who attribute longer sleep stretches to formula feeding may or may not be correct in their assessment of the situation. There are too many factors involved to easily sort out what has encouraged longer sleep or if a longer sleep stretch than would have occurred anyway has actually transpired. .
Most babies have an evening fussy spell during which they want to breastfeeding very frequently.
This is called a cluster feeding. Parents who haven't been taught about this normal behavior and strategies to deal with it frequently respond by giving a bottle of formula. This of course. Often the baby falls asleep after being offered a bottle, which also gives the parents the wrong impression that the bottle was just what the baby needed. What may have occurred is that the baby has come to the natural conclusion of the fussy spell.