Plain English Meaning

Feeling sleepy or having trouble staying fully awake and alert.

Full Definition

A state of reduced alertness in which a pilot feels sleepy, sluggish, or struggles to stay awake. In aviation, drowsiness is recognized as an early warning sign of hypoxia (insufficient oxygen reaching the brain), as well as a symptom of fatigue, dehydration, or certain medications.

Context Anchor

Seen in discussions of hypoxia symptoms, oxygen use, high-altitude flight, fatigue, and pilot fitness to fly.

Derivation

From the Old English 'drusian', meaning 'to sink or become slow.' That older sense fits the aviation meaning well — the pilot's alertness sinks before they realize it.

Why Pilots Care

It warns that the brain is oxygen-starved and can rapidly worsen to loss of consciousness or poor decisions.