" Spaced repetition makes more sense if you imagine your brain as a muscle. Muscles can’t be exercised all the time, and then put back to work with little to no recovery. Your brain needs time to make connections between concepts, create muscle memory, and generally become familiar with something. Sleep has been shown to be where neural connections are made, and it’s not just mental. Synaptic connections are made in your brain and dendrites are stimulated. "

― Peter Hollins , Learn Like Einstein: Memorize More, Read Faster, Focus Better, and Master Anything With Ease… Become An Expert in Record Time (Accelerated Learning)