Study objectives: Electroencephalographic slow wave activity (SWA) during non-rapid eye movement (NREM) sleep results from the synchronous oscillation of cortical neurons and is the standard measurement of sleep homeostasis. SWA is not a direct measure of sleep pressure accumulation, but rather a measure of the NREM-sleep response to accumulated sleep pressure. Currently, no practical standard for the direct measurement of sleep pressure accumulation exists. Recently, it was demonstrated that rat cortical neurons undergo oscillations during wake that are similar to the cortical oscillations responsible for SWA. Furthermore, these oscillations increase in number as time awake increases. Here we hypothesize that period-amplitude analysis of the electroencephalogram (EEG), which treats the EEG as a series of discrete waves, can measure these cortical oscillations, and thus, is a measure of sleep-pressure accumulation during extended wake.
Design: Mice were sleep deprived for 24 h by confinement to a slowly rotating wheel in order to assess wake-dependent changes in EEG wave incidence.
Measurements and results: Continuous period-amplitude analysis of the waking EEG across 24 h of sleep deprivation revealed that the incidence of 2 to 6 Hz waves increased exponentially over the deprivation period. This increase in wave incidence appeared to occur in two phases with exponential time constants of approximately 0.12 h and 3 h. Further analysis revealed that the changes in wave incidence were significantly correlated with two established markers of sleep pressure, SWA and NREM sleep latency.
Conclusions: The data suggest that wave incidence is an effective method of measuring sleep homeostasis in the waking EEG that provides better temporal resolution than spectral power analysis.
Keywords: Sleep deprivation; homeostasis; mice; sleep propensity; waking.