br Methods The experimental study was performed in

Methods The experimental study was performed in accordance with the ethical standards laid down in the Declaration of Helsinki. Its protocol was approved by the Ethics Committee of the Siberian Branch of the Russian Academy of Medical Sciences (#2/07.06.2009). Informed written consent was obtained from each study participant. The analyzed data set contains spectra of the EEG signals recorded from 18 male cadets of the Novosibirsk military high school. Their ages ranged from 18 to 22 years. Their regular sleep prior to the experiments occurred within 7-hour interval between the rising and bedtimes scheduled on 06:30 and 23:30, respectively. The experiments were carried out in the laboratory complex on 9 weekends, between Saturday morning and Monday morning, with 2 participants studied during each weekend. They both were kept continuously awake until 23:00, and then one of them was allowed to sleep in the sleep laboratory until 06:00h. The other was kept awake for the whole night. The next 24h were organized in 12 wake–sleep cycles each consisting of 100-minute wakefulness followed by 20-minute nap with polysomnographic recordings. A participant was asked to sleep lying in bed in a sound-attenuated and completely darkened room of the sleep laboratory during a 20-minute span with closed eyes. Then he ppar antagonist was taken out of the sleep laboratory to stay in other rooms together with experimenters for the whole time interval between consecutive napping attempts. To prevent unintended sleep, he was constantly engaged in research activities and social interactions. Polysomnographic sleep recordings were performed using a standard monitoring montage that included 5 EEG channels, two electro-oculogram channels, and one chin electromyogram channel. Data were collected via an 8-channel Medicor polygraph (EEG8S, Micromed, Hungary). Since the central derivations were recommended for visual scoring of sleep stages [20,3], the reported results are based on data from Cz-A1 derivation of the international 10–20 system of electrode placement (i.e., vertex of the head vs left mastoid). Conventional scoring of 20-min naps was performed by 2 independent judges. Thereafter, Histone deacetyltransferase ，HDAC together reexamined the epochs with discrepant scores in order to produce consensus scores. The EEG signals were high band-pass, low band-pass, and notch filtered (0.5, 64, and 50Hz, respectively), digitized at a sampling frequency of 128Hz and stored on a hard disk. The artifacts were detected at 5-second intervals, and absolute power values were computed for the artifact-free 5-second intervals using the fast Fourier transform algorithm. The spectral data were reduced to single-hertz bin widths by calculating the mean absolute power values over adjacent frequencies and by further averaging within each consecutive one-minute interval. The one-min spectra calculated for each 20-min nap were assigned relative to 0-minutes of polysomnographically determined onset of stage 1 (N1), stage 2 (N2), and stage 3 (N3). In total, 164 naps were used for the present analysis after exclusion of a few naps either containing REM sleep or showing that NREM sleep was interrupted by wakefulness. Spectra on the interval of the first 16 power values (the range from 1 to 16Hz) were log-transformed and subjected to principal component analysis. Additionally, the log-transformed power values were averaged over frequency ranges roughly corresponding to delta, theta, alpha, and sigma activities (1–4, 5–8, 9–12, and 13–16Hz, respectively). The SPSS statistical software package, version 21, was used for all statistical analyses (SPSS, Chicago, IL). Principal component analysis was run either on all sets of spectra or on the sets obtained from each of 18 participants (Fig. 1A and B). Each set was decomposed into four principal component scores. In order to calculate a score on each of the four components, the 16 original power values were optimally weighted in accord with their loadings on this component (Fig. 1A) and then summed. More details regarding the methodology of principal component analysis of the EEG spectrum have been reported earlier [11,12,17,18].