It all depends on the relationship dynamic between the individuals involved in my opinion.

Is this "too tired for an online session"?

Real life situations imho trump any such considerations, especially if one has school or needs to work the next day etc.

Is it a real life 24/7 or TPE arrangement?

If not then if one is or is not going to enter into a play session doesn't need a reason really.

Lack of consent is lack of consent in any event and should be respected.

In real life if either party is tired that should be a consideration for safety purposes.

Of course there are degrees of fatigue as well and naturally only the person who is tired can really make that judgment call.