wakeful

Wakeful is an adjective that describes the state of being awake or alert, often with a sense of vigilance. It is used to refer to people who are not sleeping and to mental states that are attentive or ready for action. In literary or historical contexts, wakeful can describe a night spent in vigil or watchfulness. The related noun is wakefulness.

The word derives from wake, meaning to emerge from sleep, with the suffix -ful, and entered English in the Middle English period.

In sleep science, wakefulness is the brain state opposite of sleep. It is sustained by arousal systems including the reticular activating system in the brainstem and thalamus, as well as orexin neurons in the hypothalamus. Various neurotransmitters promote wakefulness, such as norepinephrine, acetylcholine, dopamine, serotonin, and histamine; adenosine promotes sleep. Circadian rhythms, governed by the hypothalamic clock, modulate when wakefulness is favored during the day. Disturbances in wakefulness can manifest as insomnia (difficulty sleeping) or hypersomnia (excess daytime sleepiness).

In everyday usage, wakeful often characterizes attentiveness or vigilance, as in descriptions of a wakeful guardian or a narrator who remains wakeful through a tense night. The term remains common in literature and clinical discussions as a contrast to sleep or drowsiness.