In particular, students are generally studying materials which are unrelated to their primary classroom focus—e.g. vocabulary words for languages they’re not learning; arbitrary facts about various animal species or places or historical events; etc.
It’s not clear to what extent results established in this context will extend to naturalistic settings. This is a special case of Most experimental psychology on learning involves people learning things they don’t care about.
An important early paper establishing the Spacing effect was Spitzer, H. F. (1939). Studies in retention. Journal of Educational Psychology, 30(9), 641–656; in this paper students study unrelated materials.
This is fairly typical for the literature: roughly speaking, I’d say about 80% of the papers I read are of this kind (or are in a laboratory setting rather than a classroom setting and so are even more inauthentic).
Over time, a small number of studies have interceded directly in a course’s curriculum and tested the Spacing effect (and often also the Testing effect) in that context. These studies are probably more instructive w.r.t. implications of these effects on Knowledge work. See e.g. systematic review Agarwal, P. K., Nunes, L. D., & Blunt, J. R. (2021). Retrieval Practice Consistently Benefits Student Learning: A Systematic Review of Applied Research in Schools and Classrooms. Educational Psychology Review..
Collecting papers to read about undergraduates or adult learners (drawn from Agarwal, P. K., Nunes, L. D., & Blunt, J. R. (2021). Retrieval Practice Consistently Benefits Student Learning: A Systematic Review of Applied Research in Schools and Classrooms. Educational Psychology Review.):
last 6 months. Advances in Health Sciences Education, 15, 395–401.
explanation on long-term retention. Medical Education, 47, 7 674–682.
test-enhanced learning with standardized patients and written tests improves clinical application of knowl-edge. Advances in Health Sciences Education, 18, 3, 409–425.