Motor learning, the process of acquiring and refining motor skills, is crucial for various aspects of life. Skill acquisition, the rapid improvement observed immediately after a training session, represents the initial stage of motor memory formation. Subsequent consolidation, involving offline learning during rest periods, transforms labile motor memories into robust, long-term memories. Sleep has been extensively documented as playing a critical role in this consolidation process. While the influence of training parameters like amount, rest distribution, and skill variation on motor learning has been well-studied, the impact of the time of day has received less attention. Although circadian rhythms affect various aspects of motor performance, such as maximal voluntary contractions, spontaneous motor tempo, and speed-accuracy trade-offs, the optimal time of day for motor learning remains unclear. Existing literature presents conflicting findings; some studies report no time-of-day effect on skill learning, while others demonstrate a positive influence, particularly emphasizing sleep's role. This study aims to clarify this ambiguity by designing a rigorous experimental protocol to investigate the independent effects of time of day and sleep on both motor skill acquisition and consolidation. The hypothesis is that skill acquisition and consolidation will be superior in the afternoon compared to the morning, based on the known daily fluctuations in physiological mechanisms like cortisol secretion and hippocampus activation, both linked to motor cortex plasticity and memory consolidation.

Numerous studies have highlighted the importance of practice and rest in motor skill acquisition and consolidation. Research consistently shows that a single practice session leads to rapid performance improvement, reaching an asymptote. Further improvements are observed during rest periods, demonstrating the role of offline learning. The critical role of sleep in motor skill consolidation has been demonstrated across multiple studies, using various experimental designs incorporating both diurnal and nocturnal sleep groups. However, the optimal time-of-day for motor learning remains a less explored area, with existing studies yielding mixed results. Some studies find no time-of-day effects on skill learning, while others suggest a positive correlation, particularly emphasizing the role of sleep. This study aims to address these inconsistencies by separating the effects of time-of-day and sleep on motor skill acquisition and consolidation.

Sixty healthy, right-handed adults (excluding musicians and professional typists) participated, with 36 randomly assigned to three training groups (10 a.m., 3 p.m., and 8 p.m.), and 23 to two control groups. The finger-tapping task involved tapping a sequence of keys (1-4-2-3-1-0) with their non-dominant hand. Skill was measured by a composite ratio combining movement duration and error rate. The main experiment included pre-test (T1), training (44 trials), and post-test (T2) on Day 1. On Day 2, all participants completed a retest (T3) to assess consolidation. The 8 p.m. group's retest was 24 hours later; others were tested at different times to account for time-of-day differences in retesting. Control groups (G8sleep, G8awake) were included to isolate sleep's role; G8sleep was retested 14 hours after training (2 hours after waking), and G8awake was retested 2 hours after training. All participants were screened for chronotype and sleep quality using standardized questionnaires to control for potential confounding factors. Data analysis involved repeated measures ANOVA, one-way ANOVA, t-tests, and power-law function fitting to analyze learning curves. Bayesian equivalence tests were used to support conclusions in cases of null effects. A composite score incorporating both error rate and movement duration served as the primary measure of motor skill performance. The gain in motor skill between different testing sessions was calculated using proportional formulas.

The study's main findings revealed no significant difference in motor skill acquisition across the three training time groups (10 a.m., 3 p.m., and 8 p.m.). However, there were significant differences in skill consolidation 24 hours post-training. The 10 a.m. group experienced a deterioration in performance (p = 0.03), the 3 p.m. group showed stabilization (p = 0.71), and the 8 p.m. group showed a significant improvement (p = 0.02). Analysis of learning curves using power functions confirmed that the improvement in the 8 p.m. group was due to offline learning and not simply a continuation of the learning trend observed during the initial training phase. Control experiments further supported the crucial role of sleep in consolidation. The G8sleep group (trained at 8 p.m., retested 14 hours later after a night's sleep) showed similar consolidation improvement to the G8pm group. In contrast, the G8awake group (trained at 8 p.m., retested 2 hours later without intervening sleep) showed no significant consolidation. These findings suggest that sleep plays a critical role in consolidating motor skills when training occurs in the evening, close to sleep time. The analyses also controlled for the effects of chronotype and sleep quality, ruling out these factors as significant influencers of the observed results.

The present study's results demonstrate a clear dissociation between motor skill acquisition and consolidation, highlighting the specific influence of sleep on the latter. While acquisition proved independent of training time, consolidation varied significantly, suggesting a temporal window where sleep facilitates offline learning. This temporal proximity may protect against memory interference. The morning group's performance decline could be linked to increased exposure to interference due to extended wakefulness. The observed differences in consolidation may stem from variations in neural correlates during acquisition, impacting subsequent consolidation processes. Furthermore, the involvement of different memory systems (procedural and declarative) in the finger-tapping task might contribute to the observed time-of-day effect on consolidation; the morning group's poor consolidation may be due to competition between these memory systems. Future studies could investigate the role of napping after morning training to determine whether it can replicate the consolidation benefits seen in the evening training group.

This research provides strong evidence for the importance of timing in motor skill training, emphasizing the benefit of evening training followed by sleep for optimal consolidation. Although further research is necessary to fully elucidate the underlying neural mechanisms and generalize these findings across diverse populations and chronotypes, the study has significant implications for the planning of effective training programs in sports, clinical settings, and experimental research. Future studies should investigate the interaction between various training schedules, sleep patterns, and individual chronotypes to offer more personalized recommendations.

While the study controlled for several factors, including chronotype and sleep quality, potential limitations include the sample size and the use of a specific motor task. The findings may not be generalizable to all motor skills or populations. The study primarily focused on individuals with intermediate chronotypes, limiting the generalizability to other chronotypes. Further studies are required to explore these aspects, and to investigate the possible role of nap-like sleep interventions for the optimization of motor skill learning.