"Proved" is not a word used in any physiological research I've ever been familiar with, at least not with paragraph, if not pages of qualification. Would you please link me to this proof?
Agree, I shouldn't have used the word "proved". But there is a lot of robust evidence for it going back to the 1800s. Many of the studies are linked to on the wikipedia page
https://en.wikipedia.org/wiki/Spacing_effect