I'll say more....
It happens now as a matter of course on every single through the night flight where I work. Wasn't the case 5 years ago - but since it's been legitimized by being written in the manuals its become SOP.
But its not a fatigue issue - cause fatigue is described as long term tiredness/exhaustion - whereas this is only " unexpected tiredness on the day".
Funny that I ALWAYS unexpectedly feel tired around 4 AM....although we tend to make sure the guy/girl that's operating/awake doesn't play Ipad games at the time...