If "spring fever" describes the excitement and restlessness that comes with the arrival of spring, then its antonyms are words that might connote the opposite. For example, "apathetic," "indifferent," and "lethargic" could describe someone who doesn't feel any different in spring than they do during any other season. "Sorrowful" or "depressed" could describe someone who is not uplifted by the warmer weather or longer days. "Lazy" might connote someone who doesn't feel like getting up and doing anything, rather than the motivated energy normally associated with spring fever. In a sense, then, any word that describes a lack of energy, enthusiasm, or excitement could serve as an antonym for "spring fever.