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Introduction
Sleep and wakefulness have been traditionally considered as mutually exclusive states. At the behavioral level, sleep is characterized by a transient loss of responsiveness to the environment (Carskadon and Dement, 2005; Cirelli and Tononi, 2008). This behavioral unresponsiveness was found to correspond, at the physiological level, to specific patterns of brain activity. Notably, Non-Rapid Eye Movement (NREM) sleep, which amounts to 75–80% of the total time spent asleep in healthy adults (Ohayon et al., 2004; Carskadon and Dement, 2005), is characterized by the occurrence of high-amplitude slow oscillations. These so-called “slow waves” were initially described as alternations between moments of neuronal silencing and firing synchronized across the entire cortex (Steriade, 2003; Vyazovskiy and Harris, 2013).
Recent discoveries in animal and human sleep physiology have tempered the notion of slow waves as necessarily being a global physiological event (Nobili et al., 2012; Siclari and Tononi, 2017; Krueger et al., 2019). Within NREM sleep, some brain regions can show slow waves while others do not (Nir et al., 2011; Nobili et al., 2011). Such regional aspects of sleep activity had already been observed in certain animal species (e.g., dolphins who can enter unihemispheric sleep, sleeping in one brain hemisphere at a time) (Mascetti, 2016; Rattenborg et al., 2019) and in sleep pathologies (Terzaghi et al., 2009; Dodet et al., 2015; Castelnovo et al., 2016; Riedner et al., 2016). However, in the past few years, local sleep involving changes in sleep depth within NREM sleep has been robustly observed in individuals without sleep disorders (Huber et al., 2004; Nir et al., 2011). Even more striking is the observation of local sleep-like slow waves outside of NREM sleep, in wakefulness (Vyazovskiy et al., 2011; Hung et al., 2013; Bernardi et al., 2015; Quercia et al., 2018) or REM sleep (Funk et al., 2016; Bernardi et al., 2019). These slow waves are isolated (local in time) and spatially restricted (local in space) and consequently largely overlooked in standard classifications that focus on the global characteristics of sleep and wake states. The occurrence of local slow waves outside NREM sleep (i.e., in wakefulness or REM sleep) and the local modulation of the presence of slow waves within NREM sleep have been termed ‘local sleep’ (Box 1).
Box 1.
Glossary: Some...