Two features of sleep slow waves: homeostatic and reactive aspects – from long term to instant sleep homeostasis

• We reviewed the shifts in views toward use dependency of slow wave homeostasis and its role in plastic changes during sleep.
• We also reviewed research on reactive aspects of sleep slow waves.
• We pointed out the dynamic relationship between sleep homeostasis and reactivity features.
• An instant homeostatic function is proposed for CAP A1phase, to maintain nightly delta amount.
• Traditional and instant homeostasis provides a double safety mechanism for frontal lobe cognitive functions.

In this paper we reviewed results of sleep research that have changed the views about sleep slow wave homeostasis, which involve use-dependent and experience-dependent local aspects to understand more of the physiology of plastic changes during sleep. Apart from the traditional homeostatic slow-wave economy, we also overviewed research on the existence and role of reactive aspects of sleep slow waves. Based on the results from spontaneous and artificially evoked slow waves, we offer a new hypothesis on instant slow wave homeostatic regulation. This regulation compensates for any potentially sleep-disturbing events by providing instant “delta injections” to maintain the nightly delta level, thus protecting cognitive functions located in the frontal lobe. We suggest that this double (long-term /instant) homeostasis provides double security for the frontal lobes in order to protect cognitive functions. The incorporation of reactive slow wave activity (SWA) makes sleep regulation more dynamic and provides more room for the internalization of external influences during sleep.