Why can two brief reviews beat one long look when you need the idea next week?

Answer: The gap makes recall effortful

Breaks reduce fatigue — Breaks can reduce fatigue, so this is a reasonable surface explanation. But spacing effects appear even when the main issue is memory timing, not tiredness. The later review is useful because the idea is no longer fully warm and has to be reconstructed. Rest helps most when it sets up a harder retrieval attempt.

The gap makes recall effortful ✓ — Right. Spacing lets retrieval strength fade a little, so the next successful recall has to work harder and can strengthen longer-term access. Cepeda's review covered hundreds of experiments, and Bjork describes spacing as a desirable difficulty. The odd lesson is that a little forgetting can be useful fuel.

Attention resets each time — A fresh attention reset can help, especially when a long session becomes dull. But spacing is more than a new start: it changes the memory state between encounters. The second review arrives after the idea has cooled, so recall has to be rebuilt. That effortful rebuilding is the part a mere attention reset does not explain.