Circadian and sleep–wake homeostatic modulation of EEG activity during sleep and wakefulness

Abstract The human sleep–wake cycle is regulated by two distinct processes: the circadian timing system (CTS) and the sleep–wake homeostatic (SWH) process. The CTS is driven by a small region in the anterior hypothalamus of the brain, which is known as the “circadian clock.” By contrast, the SWH can be conceptualized as an hourglass, whereby sleep pressure builds up during waking hours and is released during sleep. In contrast to the CTS, there is no specific brain region that controls this hourglass process. A complex modulation of these two processes affects electroencephalographic (EEG) activity throughout the 24‑h day, resulting in the emergence of specific EEG features. These features can be classified into three categories: those that show clear circadian patterns, those that are predominantly influenced by the homeostatic process, and those that show a combination of both. This review describes the EEG features quantified by spectral analysis during sleep and wakefulness derived from specific human study protocols, which enable the separation of the influence of the circadian clock and the sleep–wake hourglass process. Second, the potential for circadian and SWH modulation to influence interictal activity and seizure occurrence will be discussed, along with its implications for the diagnosis, treatment, and seizure prediction and prevention.