wakefulness

Wakefulness is the physiological state of being awake, alert, and responsive to the environment. It contrasts with sleep and other altered states of consciousness and underpins attention, perception, and voluntary action. In humans, wakefulness arises from an interconnected network that sustains cortical arousal and readiness to process sensory input.

The core neurobiology involves the ascending arousal system, which originates in the brainstem reticular formation and projects to the thalamus and cerebral cortex. Key neurotransmitter systems include norepinephrine, serotonin, histamine, acetylcholine, and the orexin (hypocretin) neurons in the hypothalamus, which promote and stabilize wakefulness. The circadian system, centered in the suprachiasmatic nucleus, interacts with homeostatic sleep pressure to regulate the timing and duration of wakefulness across the 24-hour cycle. Adenosine accumulation during wakefulness contributes to sleep propensity, while light exposure helps synchronize circadian rhythms.

Assessment of wakefulness combines subjective reporting, behavioral observation, and physiological measures. EEG during wake shows low-amplitude, mixed-frequency activity; actigraphy and sleep-wake diaries track patterns, and tests such as the multiple sleep latency test (MSLT) evaluate daytime sleepiness and propensities.

Clinical relevance includes disorders affecting wakefulness, such as insomnia (difficulties sustaining wakefulness in restricted sleep periods), hypersomnolence disorders like narcolepsy and idiopathic hypersomnia, and circadian rhythm sleep-wake disorders. Wakefulness is modulated by sleep deprivation, illness, medications, and stimulants (for example caffeine or modafinil) that influence arousal pathways. Adequate wakefulness is essential for attention, decision-making, and motor performance, with disruption impacting safety and health.