The Cyclic Structure of Sleep: Relationship Between the Macrostructural Slopes of Cycles and Microstructural Dynamics

Abstract

We are dealing with the relationship between macrostructural (slope of the sleep cycles) and the microstructural dynamics. Descending (“D”) and ascending (“A”) slopes of cycles show opposite trends: “D” slope shows sleep promoting, while “A” slope shows wake/REM-promoting tendencies. Arousal-like phasic changes (PAT and CAP A_2–3 phases) turned out to be overwhelming during the “A” slopes of sleep cycles, and their amount is increased from evening to morning. Within the “A” slopes their frequency increases before REM sleep periods. Sleep-like phasic changes have a different course. They are prevalent during the “D” slopes of the first sleep cycles and follow the homeostatic decay of slow wave oscillation. Accordingly, the elicitability of the two types of phasic events has opposite tendencies during night sleep from evening to morning: Arousal-like responses are prevalent during the “A” slopes and their responsivity increases, while sleep-like responses are prevalent during the “D” slopes and their elicitability decreases. Lastly, we point out the double nature of the forces behind tonic and phasic regulation: There are alternating chemical influences behind the tonic one and faster reciprocal sleep- and wake-promoting inputs behind the phasic regulation. Due to the intermingled phasic and tonic regulation, the sleeper is connected with the surrounding world and is able to execute two seemingly contradictory tasks: (1) separation for trophotropic (use dependent homeostatic) functions and (2) reactivity in form of state-dependent responses.