Objective: There is consolidated evidence that stage changes in sleep are closely related to spontaneous EEG fluctuations centered on the 20±40 periodicity of the cyclic alternating pattern (CAP). The present investigation aimed at assessing the involvement of the different components of CAP in the process of build-up, maintenance and demolition of deep non-REM (NREM) sleep. Methods: CAP parameters were quanti®ed in the first 3 sleep cycles (SC1, SC2, SC3), selected from polysomnographic recordings of 25 healthy sound sleepers belonging to an extensive age range (10±49 years). Only ideal SCs were selected, i.e. the ones uninterrupted by intervening wakefulness and in which all stages were represented and linked in a regular succession of a descending branch, a trough and an ascending branch. Results: Among the first 3 SCs, a total amount of 45 (SC1, 16; SC2, 13; SC3, 16) met the inclusion requirements. SC1 contained the highest amount of slow wave sleep (43.7 min) and the lowest values of CAP rate (31.6%). The number of phase A1 subtypes remained unmodified across the 3 SCs (SC1, 48; SC2, 48; SC3, 48), whereas both subtypes A2 (SC1, 9; SC2, 14; SC3, 14) and A3 (SC1, 2; SC2, 8; SC3, 10) increased signi®cantly (P , 0:028 and P , 0:0001, respectively). The A1 subtypes composed more than 90% of all the A phases collected in the descending branches and in the troughs, while the A2 and A3 subtypes were the major representatives (64.3%) of the Aphases occurring in the ascending branches. Conclusions: Within the dynamic organization of sleep, the non-random distribution of CAP sequences, with their succession of slow (subtypes A1) and rapid (subtypes A2 and A3) EEG shifts, seem to be responsible for sculpturing EEG synchrony under the driving andalternating forces of NREM and REM sleep.
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