The present findings propose a new aspect of effective learning strategies for adolescents. Based on studies of overnight memory consolidation, we demonstrated that off-line gains in procedural finger-tapping were significantly increased in subjects trained in the evening directly before night-time sleep compared to subjects trained in the afternoon 7.5 hrs before night-time sleep. In contrast, declarative memories for word-pairs were preferentially retrieved after encoding in the afternoon compared to encoding in the evening.
Recent studies have shown that sleep after learning facilitates the consolidation of novel procedural and declarative memories in comparison to equal periods of wakefulness (for review see 
). Other studies have shown that these processes are, at least in part, impaired in patients with primary sleep disorders 
. Our study extends the concept of sleep-related memory consolidation by providing first evidence that the timing of learning before night-time sleep differentially affects the consolidation processes in the two major memory systems.
Our observation that motor learning directly before night-time sleep was especially effective fits to current models of a time- and sleep-dependent process of motor skill consolidation (for review see 
). The effect of sleep on motor learning has been demonstrated to be particularly high within the first 4–6 hrs after initial training – a period of enhanced susceptibility of initially instable procedural memory representations to disruptive interference 
. Furthermore, the consolidation of a new motor skill has been shown to be impaired by subsequent training of a second skill, but no disruption was observed if at least 4 hrs elapsed between the training sessions 
. Consistently, Korman et al. 
showed that the robustness to interference of a finger-opposition tapping task 2 hrs after training was significantly enhanced when subjects slept 90 min immediately after training. Sleep-specific brain activity patterns, such as sleep spindles 
or electroencephalographic slow wave activity 
, might foster this consolidation process in the procedural memory system 
. Preclinical studies have provided support for a time- and sleep-dependent consolidation process. On a cellular level, de novo protein synthesis and gene expression after training seem to be necessary for successful motor skill learning 
and have been shown to be modified during subsequent sleep 
. Taken together, these studies support our finding that procedural motor consolidation is especially effective when training takes place in the evening, directly prior to night-time sleep.
Our second main finding – the preferential overnight retention of declarative memories after encoding in the afternoon – contradicts previous studies on sleep-related memory consolidation. Specifically, Gais et al. 
reported enhanced declarative memory consolidation of vocabulary words in adolescents when sleep directly followed initial acquisition. Similarly, Talamini et al. 
showed in adults that the consolidation of declarative associative (spatial) memories was significantly higher when sleep directly followed the initial training session. These studies focused on the effects of sleep directly after learning in comparison to waking conditions with sleep delayed for longer periods of 11 hrs (Talamini et al.) and 15 hrs (Gais et al.) where learning took place in the morning or evening. Differences in homeostatic aspects (shorter interval of 7.5 hrs between acquisition and night-time sleep) and the circadian phase (acquisition in the afternoon) in the present study might explain the different effect on the development of declarative memory traces. A possible molecular mechanism for the enhanced retention after early encoding observed in the current study arises from preclinical studies showing that the time course of declarative memory is strongly dependent on the late phase of hippocampal long-term potentiation (LTP) which is known to last from 3 to 24 hrs after encoding 
. The late phase of LTP requires both gene transcription and translation, leading to the growth of new synaptic connections 
and is known to benefit from post-learning sleep 
. Thus, intense declarative learning in the afternoon might allow for pre-sleep processes of plasticity and for the coincidence of night-time sleep with a critical window of synaptic long-term plasticity in a hippocampal-neocortical network required for declarative memory consolidation.
Further analyses of our declarative findings revealed that the preferential retention of declarative memories after encoding in the afternoon was not sustained across a retention interval of 7 days (P
0.053). Furthermore, we can not exclude the possibility that slight but non-significant differences in sleep parameters between the groups might have contributed to the results. Thus, the declarative findings appear less robust than the procedural results and should be interpreted with caution.
From a translational perspective, our results may have relevant implications: it might be more effective to learn declarative material, like vocabulary words, in the afternoon and to train procedural skills, such as those required for music or sports, in the evening. Remarkably, training in the evening, compared to training in the afternoon, resulted in a significantly elevated gain in motor performance not only 24 hrs after initial training, but also at follow-up after one week. As noted earlier, our declarative findings appear less robust and should be interpreted with caution. Together, our results are informative for the development of new and potentially more effective teaching and learning strategies for adolescents, their parents and teachers.
Whereas we observed robust effects on the behavioral level, relevant questions on the potential mechanisms persist. First, it is possible that the observed effects were not driven by sleep-related factors, but result, at least in part, from differences in the circadian phase of the encoding and retrieval sessions (afternoon group 3 pm
, evening group 9 pm
). In our study, baseline parameters for acquisition, attention and working-memory were similar in both experimental groups. However, to further disentangle sleep-dependent from circadian effects, it would be necessary to additionally study an afternoon and evening control group under conditions of total sleep deprivation. Second, differences in memory consolidation might arise from different levels of stimulus interference across the retention interval. Importantly, in our study, retrieval was assessed 24 hours and 7 days after acquisition in both the afternoon and the evening group and the level of interference was kept comparable during wakefulness. Thus, the timing of sleep or the circadian phase rather than differences in interference explain our findings. It is important to note, however, that different aspects of the memory tasks itself can have interfering effects on the consolidation in the respective memory system. There is some evidence showing that reciprocal interactions between memory systems might occur during wakefulness but might be processed independently during sleep 
. Finally, one should bear in mind that our findings are restricted to female adolescents. All participants demonstrated post-pubertal status and were strictly investigated in their follicular phase of the menstrual cycle. Whether our findings might translate to male adolescents and potentially also to adults remains unclear. Additional studies are needed to investigate the effects of the timing of learning before night-time sleep on memory consolidation in these populations.
In conclusion, our results indicate that learning directly before night-time sleep preferentially promotes procedural memory consolidation, whereas – with less confidence – learning in the afternoon, 7.5 h before night-time sleep, might provide better conditions for the consolidation of declarative memories in adolescents. Even though it should be borne in mind that this is the first study showing these results, the findings might contribute to the development of new effective teaching and learning strategies. Translating the results to the every-day life of adolescents, we propose that declarative memories, such as vocabulary words, should be studied in the afternoon and motor skills, like playing soccer or piano, should be trained in the late evening. Most parents among us would have preferred the opposite results.