Crew Fatigue: Put Light Exposure into the Equation

Time-zone differences are a convenient way to describe a journey. Understanding how a crew member’s body clock responds also requires knowing when they encounter light and darkness.

A new study by Huang and colleagues, Modeling the effects of flight itinerary on jetlag duration, makes that distinction particularly relevant. Across 55,296 simulated flights, departure time and flight duration influenced predicted jetlag duration - even for the same time-zone difference. Some itineraries also changed the direction of circadian adaptation. The authors attribute these effects mainly to light exposure and sleep timing during and around flights.

For aviation, this suggests substantial potential to improve biomathematical fatigue models (BMMs) where they rely heavily on time-zone differences or simplified assumptions about acclimatization. Incorporating more realistic light conditions before departure, onboard and after arrival could better represent the signals driving the crew’s body clock. Airport lighting, the cabin environment, daylight during transfers and darkness during rest all deserve consideration.

The practical next step is to model those exposures and test whether incorporating them improves predictions of crew sleep and alertness. The study provides a strong rationale, although its simulations use standardized lighting and group-average physiology; it does not establish the size of any improvement in operational BMMs.

The same reasoning should inform fatigue regulations. Where acclimatization and recovery provisions depend heavily on time-zone displacement and elapsed time, light exposure deserves attention as an additional physiological factor. Regulatory development should support validated approaches that capture this influence.

One finding is especially relevant to safety: forcing daytime wakefulness at the destination sometimes shortened predicted jetlag while worsening predicted vigilance initially. Faster adaptation therefore cannot automatically be treated as better readiness for duty.

For crew fatigue management, the promising question is how much better we can predict recovery when we account for the light experienced throughout the journey.