FORMULA OR BREAST ?

Shay • Living life on the hippy side , I run an animal sanctuary for exotics and other helpless unwanted critters . Married mom of two wonderful girls . Not a crazy crystal lady but heavy into mysticism and traditional plant medicines 💗 . Free spirited and poly
So I formula feed at breast feed , I breast feed with a nipple shield so baby doesn't have nipple confusion , and she latches better ! But why is it that when I breast feed she gets really sleepy and falls asleep after but when I feed her formula she's up thru the whole thing and gets sleepy later and wakes up sooner ??