You don't need an excuse. If you're at a party and no longer want to be there, say "I have to go now, thanks for the great party" to the host and leave
No one cares why you're leaving. I'm probably well on the neurotypical end of the spectrum and I'm usually there first in my friend group to leave a party just because parties are late and I wake early. No one complains unless I'm their lift home and they don't want to leave yet