Embracing sleep onset complexity

Abstract:

Sleep is crucial for many vital functions and is extensively studied. In contrast, the sleep onset period, often portrayed as a mere prelude to sleep, has been largely overlooked and remains poorly characterised. However, a recent series of studies has reignited interest in this transitional period, shedding light on its neural mechanisms, cognitive dynamics, and clinical implications. This review synthesises the existing knowledge on the sleep onset period in humans. We first examine the current definition of the sleep onset period and its limits, considering the dynamic and complex electrophysiological changes that accompany the descent to sleep. We then describe how internal and external processing interplay during the wake-to-sleep transition. Finally, we discuss the putative cognitive benefits of the sleep onset period, its scientific value, and identify novel directions to better diagnose sleep onset disorders.