Sleep-independent off-line enhancement and time of the day effects in three forms of skill learning

The role of sleep in memory and skill-learning processes is an important and widely debated issue. The current study explores the nature of the relationship between sleep and off-line improvement in three tasks for measuring different aspects of skill learning: the serial reaction time (SRT) task, which is a motor sequence learning task; the artificial grammar learning (AGL) task, testing abstract verbal sequence learning; and the weather prediction (WP) task, which is a non-sequential categorization task. Each participant was tested on one of the three tasks twice, either in a Wake condition (with a 12-h off-line period without sleep), or in a Sleep condition (with sleep). Results showed no sleep-related off-line improvement throughout the three tasks in a two-session re-learning design, but a sleep-independent time-based effect was found on the SRT task. No performance boost was observed in the WP and AGL tasks. Performance on the SRT showed a time of the day effect: the Sleep group outperforming the Wake group; however, this effect was restricted to overall response latencies. Taken together, no evidence was found in favor of sleep-dependent off-line enhancement in skill learning, but methodological concerns warrant further investigations.