Abstracts

Rationale: The non-rapid eye movement (NREM) sleep is considered a true resting state for the brain, yet the physiologic drivers of neural activity in this phase remain undetermined. Neural activity may be endogenously driven to compensate for minimal sensory stimulation, or by physiologic processes associated with development, or the autonomic tone. The relationship between sleep structures and autonomic function has been investigated for several decades and it is well established by now that K-complexes but not spindles are associated with transient changes in cardiovascular and sympathetic function, However, those studies have focused on fast time scales, that is, on transient interactions between brain activity (i.e. K-complex waveform) and autonomic changes, and have ignored tonic effects on larger time scales, that is, the relationship between baseline autonomic tone and baseline EEG activity. Here, we have investigated these tonic effects by focusing on the spindle and K-complex counts over long periods of uninterrupted N2 sleep, and demonstrated that they are uncorrelated to autonomic parameters after adjusting for natural co-variations of all of these variables with age. We thus conclude that tonic neural activity during N2 sleep, quantified as the densities of sleep and K-complexes, increases progressively with age during development into adulthood, but is unrelated to the baseline autonomic tone. Methods: We examined brain activity and autonomic tone during stage 2, NREM sleep over a cohort of 93 subjects, from toddlers to young adults. Brain activity was quantified as total counts of EEG spindles and spontaneous K-complexes over 30 minutes of uninterrupted sleep. Autonomic tone during sleep was measured as baseline heart rate and amplitude of respiratory sinus arrhythmia. Blood pressure was also measured at rest. Results: On average, spindle and K-complex counts increased by 8 and 1 units, respectively, with each additional year of development. Furthermore, spindle and K-complex densities were strongly correlated even after adjusting for age, which suggests a common underlying mechanism. Across subjects, spindles were on average 15 times more abundant than spontaneous K-complexes. Gender did not significantly influence any of these trends. Regarding autonomic tone, spindles and K-complex densities were anti-correlated with baseline heart rate and positively correlated with blood pressure, but these correlations are explained by natural co variations with age. Finally, neither respiratory rate nor sinus arrhythmia correlated with spindle or K-complex density, after adjusting for age. Conclusions: These results collectively demonstrate that spindle and spontaneous K-complex densities increase progressively with age during development into adulthood, and are unrelated to baseline autonomic tone. Funding: This work has been supported by The Hartwell Foundation