Somnolence

Somnolence is a state of pronounced sleepiness in which staying awake is difficult. It ranges from mild drowsiness to near-sleep and can arise from normal sleep pressure, illness, or medication.

Sleep homeostasis: The regulation that increases sleep drive during wakefulness and reduces it during sleep. Its accumulating pressure is a major source of somnolence after prolonged wakefulness.
Excessive daytime sleepiness: Persistent or recurrent sleepiness during waking hours that interferes with daily life. This clinical symptom is somnolence that becomes excessive or functionally disruptive.
Epworth Sleepiness Scale: A questionnaire that estimates the likelihood of dozing in common everyday situations. It provides a standardized way to quantify sleepiness associated with somnolence.
Fatigue: A subjective sense of physical or mental exhaustion that may occur without a strong tendency to sleep. Fatigue can feel draining without the sleep propensity that defines somnolence.
Circadian rhythm: An approximately 24-hour biological cycle that coordinates bodily functions with day and night. Circadian timing can produce predictable peaks in sleepiness even after adequate sleep.
Narcolepsy: A chronic neurological disorder marked by excessive daytime sleepiness and, in some cases, cataplexy. It is a well-known disorder in which severe somnolence can occur despite adequate sleep.
Multiple Sleep Latency Test: A daytime test that measures how quickly a person falls asleep across scheduled nap opportunities. It objectively assesses sleep propensity when excessive somnolence needs investigation.
Lethargy: A state of low energy, reduced activity, or diminished responsiveness. Lethargy describes reduced engagement, whereas somnolence specifically involves sleepiness.
Adenosine: A signaling molecule whose accumulation in the brain during wakefulness contributes to sleep pressure. Its wake-dependent buildup helps explain increasing somnolence over the day.
Obstructive sleep apnea: A disorder in which repeated upper-airway obstruction disrupts breathing during sleep. Fragmented nighttime sleep commonly leaves affected people somnolent during the day.

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