CAP time is the temporal sum of all CAP sequences. CAP time can be calculated throughout total NREM sleep and within the single NREM stages.
The percentage ratio of CAP time to sleep time is referred to as CAP rate. CAP rate can be measured in NREM sleep (percentage ratio of total CAP time to total NREM sleep time) and in the single NREM stages (percentage ratio of CAP time in a given stage to the total duration of that stage throughout sleep). In human sleep, CAP rate is an index of arousal instability that shows a u-shaped evolution along the life span (teenagers, mean 43.4%; young adults, 31.9%; middle aged, 37.5%; elderly, 55.3%) (Parrino et al, 1998) and correlates with the subjective appreciation of sleep quality (the higher the CAP rate the poorer the sleep quality) (Terzano et al, 1990; Terzano and Parrino, 1992). When the increases of CAP rate are not accompanied by relevant variations of the sleep stages (macrostructure), it means that the flexibility of the microstructure (CAP) protects the stability of the macrostructure. However, because CAP rate is a dynamic parameter that measures the effort of the cerebral centers to maintain a compatible organization of sleep, an excessive enhancement of instability drags a fragmentation of stage sequences, with a drastic curtailment of the deeper ones and an increase of nocturnal awakenings. Thus, the oscillating instability of CAP acts both as a protective mechanism for architectural stability and as a preparatory device for macrostructural alterations.
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