alertness has 6 distinct synonyms spread across 3 of its 3 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
the process of paying close and continuous attention
“wakefulness, watchfulness, and bellicosity make a good hunter”
a state of readiness to respond
“alerting was indicated by the desynchronization of the EEG”
Synonyms, antonyms and the broader and narrower relations for alertness come from Princeton University’s WordNet. Full sources and licences.