From the Australian Civil Aviation Safety Authority website;
The FAA (2012, p. 359), reference to a 13 hour FDP could result in periods of wakefulness in excess of 16 hours before their FDP resulting in a human performance similar to that of an individual over the legal limit for alcohol consumption.
• In their study quantifying the performance impairment associated with fatigue Lamond and Dawson (1999) identified that being awake for approximately 17 hours has a similar impairment on many aspects of performance as having a Blood Alcohol Concentration (BAC) of 0.05%. Being awake for approximately 21 hours has a performance impairment equivalent to 0.10% BAC.
We need to be aware of how long we will have been awake for when our duty ends.