Antonyms of lethargy:
agitation, disturbance, feeling, sensibility, sympathy, alarm, eagerness, frenzy, sensitiveness, turbulence, anxiety, emotion, fury, storm, vehemence, care, excitement, passion, susceptibility, violence, distress, enthusiasm, keenness, spiritedness, ambition, enterprise, vigor, vim, vitality, vivacity.
Examples of usage:
  1. A sort of deadness and strange weariness seemed to brood in the air, as if the great monster were in a sinister and heavy mood, full of an almost malign lethargy.
    "The Way of Ambition", Robert Hichens.
  2. It had thrown off for a moment the lethargy of business depression.
    "The Co-Citizens", Corra Harris.
  3. When this lethargy has lasted three or four months, death comes- with a tread that the patient cannot hear, and makes the slumber but a little more sound.
    "Journal of an African Cruiser", Horatio Bridge.