None of them habitually napped during the day. Before the experimental sessions, all subjects were familiarized with the experimental setting by taking an adaption nap Trichostatin A in the sleep laboratory (including electrode placement). Subjects who showed no SWS in the adaptation nap were not included in the experiment proper. The experimental protocol was approved by the ethics committee of the University of Lübeck, and the study was conducted in accordance with the 1964 Declaration of Helsinki. All participants gave written informed consent prior to participation. The experiment proper consisted of two sessions (within-subject cross-over

design), balanced in order across subjects, and separated by ~4 weeks (30.75 ± 10.5 days, to diminish carry-over effects between sessions and to control for the female menstrual cycle). In each session, participants were asked

to take an afternoon nap and perform on various learning tasks after the nap. In one of the two sessions, tSOS was applied during the nap, whereas in the other session, which served as control, sham stimulation (with an equal set-up but no stimulation) was applied. See Fig. 1A for the experimental procedure. On experimental days, subjects were required to get up at 05:00 h (to increase sleep propensity), and this was controlled by an ActiWatch 7 (CamNtech, Cambridge, UK) that was attached to the subject’s wrist (of the non-dominant hand on the evening before the session at 20:00 h), and by protocols of day-time activities. Subjects arrived at the laboratory at 14:00 h, were prepared for polysomnographic JQ1 research buy recordings and tSOS, and went to bed at 15:00 h. tSOS (or sham stimulation) began after subjects had attained stable non-REM sleep for the first time after sleep onset (see below for stimulation parameters). Subjects were woken after either one non-REM–REM sleep cycle (i.e. at the end of the first REM sleep phase) or after 90 min of sleep. After a period of 30 min, to allow recovery from sleep inertia, the enough encoding phase started; this included learning on three declarative tasks (pictures, word pairs,

and word list) and one procedural task (finger sequence tapping), which always were performed in the same order between 17:00 h and 19:30 h (Fig. 1A). A constant order of tasks was employed to reduce performance variability and because we did not expect any task interactions that would change the direction of tSOS effects. After learning, a standardized meal was served, and this was followed by the retrieval phase ~30 min later. To control for potential confounding influences of changes in arousal, mood, motivation, and activation, the Positive and Negative Affect Scale (Watson et al., 1988) was applied before sleep and before the encoding phase. To additionally control for potential differences in sleep debt, the Stanford Sleepiness Scale (Hoddes et al.