I'm not sure the above post is entirely correct because as previously mentioned I fly with some very old pilots who are very experienced, however there are some problems developing such as;
Going to sleep with no notice:
Temporary loss of concentration:
Being given a blank stare when asked to do a non-routine task they're not expecting.
Checking out their ticker is fine but how do you check what's going on between their ears?
When these guys have their medicals it's not 0300 and 9 hours into a flight and they've had a few days off.