Definition

Time in the circadian body clock cycle when fatigue and sleepiness are greatest and people are least able to do mental or physical work. The WOCL occurs around the time of the daily low point in core body temperature - usually around 0200-0600 when a person is fully adapted to the local time zone. However, there is

individual variability in the exact timing of the WOCL.

Source: ICAO Fatigue Management Guide for ATS Provders

Description

The circadian body clock is one of two key processes that regulate sleep timing and quality (the other is the sleep homeostatic process, i.e. the body's need for deep sleep that builds up during waking and discharges during sleep). The circadian body clock has connections to sleep-promoting and wake-promoting centres in the brain, which it modulates to control the sleep/wake cycle.

The Window of Circadian Low (WOCL), which occurs around the time of the daily minimum in core body temperature, corresponds to the time of day when people feel most sleepy and are least able to perform. The circadian body clock and the sleep homeostatic process interact to produce two times of peak sleepiness in 24 hours. For most people on a normal routine with sleep at night these occur:

• Between 3 and 5 a.m. Sleepiness is greatest when people are awake during this WOCL.
• In the earky afternoon (around 3-5 p.m.). Sleepiness increases again in that period(sometimes called the afternoon nap window). Restricted or disturbed sleep at night makes it harder to stay awake during this period.

The precise timing of the two peaks in sleepiness is different in people who are morning types (whose circadian rhythms and preferred sleep times are earlier than average) and evening types (whose circadian rhythms and preferred sleep times are later than average). Across the teenage years, most people become more evening-type. Across adulthood, most people become more morning-type. This progressive change towards becoming more morning-type has been documented in flight crew members across the age range 20-60 years.

Due to the inherent 24/7 nature of the business, most aviation personnel (including pilots, air traffic controllers and maintenance personnel) is exposed to work within the WOCL. A properly operating FRMS can mitigate the associated risks by instilling policies and procedures such as:

• Increased awareness of the issue, e.g. by including relevant content in the initial and/or refresher training.
• CRM/TRM training (either dedicated sessions or as a part of the refresher training) which includes techniques for fatigue management and improved teamwork (e.g. promotion of open communication about fatigue levels).
• Ensuring of controlled rest periods, especially on long haul flights or during night shifts. These allow the personnel to take short naps (20-30 min) during specified periods so that their vigilance is restored.

Related Articles

• Fatigue
• Vigilance in ATM
• Sleep Inertia
• Micro-sleep
• Circadian Rhythm Disruption
• Circadian Rhythms (OGHFA BN)
• Fatigue Risk Management System (FRMS)

Further Reading

• Fatigue Management Guide for Airline Operators
• Fatigue Management Guide for ATS providers