This glossary is written for virtual airline and flight simulation use within Virtual Air Canada Airline. It is not intended as a real-world aviation reference.
WOCL
Window of Circadian Low
The Window of Circadian Low is the period of the 24-hour circadian cycle during which the human body experiences its lowest levels of alertness, cognitive performance, and physical readiness. For most people on a normal sleep-wake cycle, the WOCL falls between approximately 02:00 and 06:00 local time. During this window, core body temperature is at its daily minimum, melatonin levels are still elevated, and reaction time and decision-making capacity are measurably degraded. These effects occur regardless of whether the individual feels subjectively tired, making the WOCL particularly hazardous from a safety perspective.
In aviation, the WOCL is a central concept in fatigue risk management. Regulatory frameworks such as EASA Subpart FTL and FAR Part 117 impose restrictions on flights scheduled through the WOCL, including reduced maximum flight duty periods, earlier reporting requirements, and mandatory rest provisions. Airlines operating long-haul red-eye routes that transit the WOCL typically augment their crew complement to allow in-flight rest in supernumerary crew rest facilities. Controlled rest on the flight deck - a structured short sleep taken by one pilot while the other maintains watch - is also approved under some operator procedures as a mitigation for WOCL fatigue. Flight data and incident analysis consistently show increased error rates and reduced performance during WOCL-affected operations.