Breast milk vs formula making baby sleep

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So my son is a couple days shy of 4 weeks, and he’s going through a growth spurt. When I nurse and he has breast milk, he wants to eat every hour-hour and a half, and a lot of the time he won’t sleep in between feedings. But, if I give him formula, he will eat every 2-3 hours and sleep really well in between feedings. Does anyone know why this is? I know breast milk gets digested faster than formula, but is it really THAT much faster? Why does my baby sleep so well after formula and not breast milk??