Couldn't do it / wouldn't be able to do it

Hi,
To express the idea of "not beeing able to do something" can we say "couldn't do it"...I give you an example:

(situation: people are at a party)
- Richard has to get up at 5am tomorrow!
- After this party I wouldn't be able to get up so early!!

Can we replace the second sentence (without changing the meaning) by:
-After this party I couldn't get up so early!! (I'd say that this sentence has not the same meaning, but Im not sure...)

Thanks for your help