and crews need to know the difference between fatigue and sleepiness e.g. the need to sleep
That's old school thinking right there. Sleepy? Tired? Fatigued? Due to short term sleep loss? Due to interrupted sleep? Due to long term sleep loss? Doesn't matter anymore, what matters is the degree to which your cognitive and physical abilities are effected. The whole ' you're not fatigued you're just tired' mantra is yesterday's thinking. Keep up Twiggy.