Question to parents, why is breastfeeding so exhausting to moms?

Hi, I want to become pregnant in 2 years. And I never wanted kids until now. I want to be prepared as much as I can and just now I read that breastfeeding is very exhausting for moms. I was wondering why that is. Is it because of the broken sleeping schedule? Or does it tire your body when you share your milk with the child? Is formula an alternative that will tire you out less? Please explain to me because I don't know anything yet.