Sleep is not passive rest. 
Sleep is when the brain decides what to keep (Rasch & Born, 2013).

When a child falls asleep, the brain begins memory consolidation (Fenn et al., 2003).

In simple terms, the brain sorts through the day, keeps what matters, strengthens certain memories, and decides what to forget. Language and vocabulary is one of the things it works on most during sleep (Davis & Gaskell, 2009).

This is why using bedtime stories to teach kids Spanish vocabulary is so effective.

A useful way to picture this is to imagine bedtime stories as placing Spanish words neatly into labeled boxes.

During sleep, the brain files those boxes onto the correct shelves and connects them to other related knowledge.

The next day, the child can access those Spanish words more easily, even if they cannot yet explain or produce them (Gais et al., 2006; Davis & Gaskell, 2009).

Sleep has an especially strong effect on strengthening children’s memories.

Research comparing children and adults shows that children’s brains consolidate declarative memories, including words and meanings, more powerfully during sleep than adult brains do (Backhouse et al., 2008).

This is one reason bedtime Spanish stories are so effective up to around age ten, and still helpful for older children and teens who read independently before sleep.

Bedtime stories are powerful Spanish learning tools.

Researchers found that words embedded in meaningful stories, not isolated words or lists, are easier to learn and remember (van den Broek et al., 2022).
They found that reading stories builds a solid Spanish vocabulary even without using any vocabulary drills, lessons, or flashcards. 

A large 2025 meta-analysis by Sangers et al., found that reading extensively helps improve vocabulary, reading fluency, comprehension, writing, speaking, and overall language ability.

A 2024 study by Venkanna and Pavani, found that even teaching grammar through short stories works better than traditional grammar exercises.

In conclusion, the more Spanish or bilingual bedtime stories you read to a child, the faster they'll learn Spanish, without effort or setting aside special study time. Simply switch English only bedtime stories for bilingual or Spanish ones.