To what extent is being tired/in a bad mood an excuse?

[JaneFonda]

Not for anything awful, just being quite snappy and exasperated with the other person.

Just curious as to what you'd accept in your relationship.

I suppose it depends why the person is tired. If they have been up in the night/early morning with toddler/baby then fair enough but if they were just up late or hungover then they would get far less understanding from me.

Depends on context and frequency?

Frequency would be where I would look too. Also, if they're that tired is going for a sleep an option. Someone being tired and grumpy when there is no reason not to do something about it wouldn't get any sympathy really. Children get tired and grumpy and need to be told to do something about it. Adults really should sort it out.