Are blackout curtains good for nurseries and baby sleep?

Verified parenting coverage (parents.com) and parenting subreddits like r/BabyBumps and r/sleeptrain report that darkening a nursery can help babies fall and stay asleep, and several parents describe rooms going pitch black even on sunny afternoons. That said, some r/sleeptrain and r/moderatelygranolamoms voices argue wake-window timing matters more than perfect darkness, so treat blackout curtains as a helpful tool rather than a guarantee.