What do naps add, and how long should a study nap be? (Boukhris et al., 2020)

A longer nap that reaches deeper sleep helps most — Boukhris et al. (2020) found a 90-minute nap beat both a 40-minute nap and no nap.

A 90-minute nap is long enough to cycle through the deeper stages (including slow-wave sleep) where consolidation happens, which a short nap largely misses. That's why the 90-minute condition outperformed the 40-minute one on later performance in the Boukhris et al. study.

But keep the role in proportion: naps supplement night sleep; they don't replace it. A daytime nap can top up consolidation and reduce fatigue, yet it can't substitute for the full sequence of sleep cycles you get overnight. Nap as a bonus, not as a way to buy back lost hours.