Functional connectivity drifts during sleep as a marker of fluctuations in the level of consciousness

During the wake–sleep cycle, consciousness waxes and wanes, and this is thought to be reflected in varying levels of integration between brain areas. Recent studies challenged the notion that consciousness is homogeneously present or absent in a brain state, as exemplified by conscious reports found in otherwise unconscious Non-rapid eye movement (NREM) sleep. Here, we tested if functional connectivity (FC) between neurons varies within brain states in a way compatible with a fluctuating level of consciousness. We examined directed FC between neurons across the wake–sleep cycle in rats, at a scale of a few seconds. We observed that NREM sleep contains epochs in which patterns of inter-areal FC are comparable to those observed in wakefulness and REM sleep, and vice versa. Thus, circuit-level connectivity patterns are not univocally determined by the brain state in which they occur, but could rather reflect other factors such as the fluctuation in the level of consciousness that takes place not only between but also within brain states.