Figure 1.
Study design. In the afternoon group, 25 subjects were trained at 3 pm, followed by standardized activities (e.g., card games, movies).
In the evening group, 25 subjects were trained at 9 pm. The experimental sleep-laboratory night (10:30 pm to 7:30 am) followed a first adaptation night in the sleep laboratory. Parameters of memory consolidation were assessed across retention intervals of 24 hours and 7 days.
Table 1.
Results declarative word-pair task.
Figure 2.
A. Subjects in the afternoon group showed a significantly higher retention rate of correctly recalled word-pairs in the retention interval from Day 1 to Day 2 compared to the evening group. B. In the retention interval from Day 1 to Day 8, the difference tended to persist but slightly failed to reach statistical significance (P = 0.053). Error bars depict SE. The asterisk (*) indicates statistical significance, P<0.05.
Table 2.
Results procedural finger-tapping task.
Figure 3.
Procedural finger-tapping task.
A/B. Subjects in the evening group showed a significantly greater improvement in finger-tapping speed across the first (A) and second (B) retention interval compared to the afternoon group. C/D. Subjects in the evening group showed a greater gain in accuracy (as indexed by a decrease in the error rate) across the first (C) and second retention interval (D). Error bars depict SE, the asterisks (*) indicate statistical significance, P<0.05.
Figure 4.
Time course of finger tapping speed across the baseline training session, first (24 hrs) and second (7 days) test session.
As depicted, the evening group showed higher off-line gains in performance from the first to the second as well as from the second to the third test session compared to the afternoon group.
Table 3.
Polysomnographic sleep parameters.
Table 4.
Study sample.
Figure 5.
To test declarative memory consolidation, the word-pair association task was used in an adapted version consisting of 46 related word pairs presented randomly on a 15 inch computer screen for 5000 ms, followed by a 100 ms blank screen using the Presentation® software (word-pair list and procedures were identical to the ones used by [18]). Four additional word pairs at the beginning and end of the task served to buffer primacy and recency effects. The word pairs were presented repeatedly until the subject remembered at least 60% in a cued-recall test, i.e. stating the word matching the first word of the previously learned word pairs. Variables measuring declarative memory encoding on day 1 (baseline) were the number of trials to criterion as well as the number of correctly retrieved word pairs in the last trial. Memory consolidation was calculated as the percentage of correctly retrieved words at recall referred to the number of correctly encoded words in the learning session (retention rate, %). Cued-recall was performed one single time without further learning 24 hrs and 7 days after initial acquisition and the number of correctly recalled words was assessed.
Figure 6.
Procedural finger-tapping task.
The task required the subjects to repeatedly tap a five-element number sequence (4–2–3–1–4) on a keyboard with the fingers of the non-dominant hand as fast and accurately as possible for 30-s epochs interrupted by 30-s breaks. To keep working memory load at a minimum, the numeric sequence was always displayed on the computer screen while tapping. A key press resulted in an asterisk displayed under the corresponding numeric character. For each 30-s trial the speed (number of correctly completed sequences) and accuracy (error rate, defined as the number of errors relative to the total number of tapped sequences in percent) was calculated. To reinforce optimal performance, the speed and error rate were displayed to the subjects after each 30-s trial. To familiarize participants with the task, five sequences were performed prior to the test session (not included in the analysis). Each test session consisted of twelve 30-s trials. The level of performance for each session was calculated as the averaged speed and accuracy across all trials.