My own understanding of the 40day rest.
In my native country Kenya, the Postpartum period is initiated in the 8th month when mother-to-be starts nesting and slowing down. The female lineage of Aunts, cousins, mothers would gather round and talk at length about what is to come. She will be encouraged with stories, tips, advice and really get a feel of support for the coming transition. The focus is on the mother and what she would need in the postpartum period. Her duty after birth is to only feed the baby and recover. There is always someone around to cook, help change the baby, treat the mother with traditional herbs on a 'lesso' which is then tied over the hips for support (Similar to the Indonesian Benkung) and to assist with the recovery of the uterus, songs are sung, food is cooked and shared and the mother is celebrated amongst other women. Traditionally we would stay away from the outside world, so in confinement for 40 days after birth to recuperate, re-energize and stabilize ourselves after the birth. The only contact is with the immediate family liing in the home with the new mother and baby. The baby is not exposed to outside energies for fear of diseases and bad spells too. In Africa it is believed that babies are pure and evil spirits are attracted to them, so you may find a newborn baby with a drawing on her forehead to waide away the bad spirits, or mothers using bottle covers to cover their babies food away from people's eyes
This practice has been going for all the times i can remember. My own mother came to the Netherlands to honor my 40day period with both my pregnancies and births. Culturally and Traditionally, we name the baby on the 40th day and the hair is shaven to pace way for a new beginning and then everyone is welcome to hold the baby etc. Therefors; I understand first hand the importance of rest after birth
So traditionally i was raised with the culture and it is imprinted in me naturally :-)