Take a look at the wikipedia articles for Desirable Difficulties and Spacing Effect. Spacing and Interleaving was the hot topic in research the last time I loo

Take a look at the wikipedia articles for Desirable Difficulties and Spacing Effect. Spacing and Interleaving was the hot topic in research the last time I looked (about 5 years ago). Note that "spacing" is different than "spaced repetition". There are quite a few papers linked to the Wikipedia articles, though, very unfortunately the "desirable difficulties" page only lists the original papers. If you google around, you should be able to find quite a few more. Keep in mind that most of the research in this area is psychological. It is often very difficult to build good studies. On top of that, many papers have pretty poor statistics in them, so take whatever you read with a grain of salt. It's very, very easy to suffer from confirmation bias because there are a lot of studies and many of them are not great. You can pick your conclusion and build an impressive looking list of references. Just be a bit careful of that when you are going through the review literature.