Fatigue risk is assessed for every pairing which includes assumptions about sleep. This tool includes actual sleep to further assess risk and may help point to potential mitigation measures should the fatigue risk be excessive.
Basic sleep research indicates the best time for good and sufficient sleep is between 2200 and 0800 local time. Acceptable performance does occur after 6 hours of sleep assuming less than 12 hours of duty. One hour of sleep should enable 2 hours of wakefulness.
Less than 4 hours of night sleep impairs wakefulness. At least 2 consecutive nights are needed to determine utility. Daytime-only sleep is less restorative than night.
Two hours awake prior to duty start is normal. More than 2 hours may become relevant for extended duty. Up to 6 hours of wakefulness prior to duty with up to 10 hours of duty still remains below the 16 hours of acceptable performance. Fatigue impairment is higher than recommended when a combination of wakefulness and duty time exceeds 16 hours.
Additional workload factors or high task demands like a special airport should be avoided when duty time exceeds 10 hours without mitigation.
A circadian disruption occurs when sleep happens more than 4 hours from the normal sleep schedule. The effect is more pronounced after 2300 local time. Expect strong performance impairment when operating through the window of circadian low (WOCL).
The number of flights impact fatigue. Fatigue is impacted after 2 flights, but alertness should be acceptable up to 4 flights. Three or more flights at night negatively impact fatigue. Dealing with aircraft MELs and/or adverse weather also negatively impacts fatigue.