Nap timing makes a difference: Sleeping sooner rather than later after learning improves infants' locomotor problem solving.

Twenty-nine newly-walking infants who had recently given up crawling trained to navigate a shoulder-height, nylon tunnel to reach a caregiver waiting at the other end. Infants in the Nap First group napped within 30 min of initial training. Infants in the Delay First group napped four hours after training. All infants were retested six hours after training on the same locomotor problem. Learning was measured by the number of training prompts required to solve the task, exploration, and time to solve the problem. Nap First infants benefited the most from a nap; they required fewer training prompts, used fewer posture shifts from training to test, and solved the task faster compared to Delay First infants, suggesting that optimally timed sleep does not merely protect against interference, but actively contributes to memory consolidation. This study highlights the importance of nap timing as a design feature and was a first step towards limit-testing the boundaries of the relation between sleep and learning. Infants' fragile memories require regular consolidation with intermittent periods of sleep to prevent interference or forgetting.