“Subjective reporting of being tired or fatigued is very unreliable (Rosekind, et al., 1999). Research has shown that even when subjects are demonstratively performance impaired due to fatigue, most will report that they feel fine.”

“Even mild sleep restriction (decreasing time in bed from 8 to 7 hours each night for an actual loss of 40 minutes of sleep/night) degrades performance over days (Belenky, et al., 2003). This research supports the hypothesis that even after one to two hours of less rest then required, individuals will demonstrate a performance decrement. It is important to understand that despite the reduction in sleep and observable performance decrement, most subjects will not report feeling tired or fatigued. Subjects that report feeling fatigued and tired usually do so long after a significant reduction in performance occurs (Rosekind, et al., 1999).”

“Several researchers have shown that the cognitive impairment that occurs as the result of prolonged wakefulness can be equated to alcohol impairment. For example, Dawson and Reid (1997) showed that the cognitive impairment and performance decrement from 17 hours of wakefulness equates to a blood alcohol concentration (BAC) of 0.05%”

“Body temperature increases and peaks in the early to mid-evening (about 1800 hours) corresponding to the point with the lowest sleep propensity. Simply put, during this time, it is very difficult for humans to sleep. In fact, this period in the circadian rhythm has been described as a forbidden zone for sleep (Lavie, 1986). Performance follows core body temperature with an approximately two-hour lag. Thus performance is lowest between 0600 and 0800 hrs and peaks at approximately 2200 hrs. Performance and sleep propensity are thus near-mirror opposites (Belenky, 2007).”

There is a subtle yet very distinct difference between asking a crew if they want to extend vs. extending them and forcing them to call fatigued if they’re unsafe. It’s the same reason our Captains are taught to ask for input from the First Officer before stating what they think should be done.