Opposite words for lethargy:
alarm, anxiety, agitation, keenness, care, ambition, frenzy, vivacity, violence, distress, emotion, fury, vehemence, enterprise, vim, sensibility, susceptibility, sensitiveness, eagerness, passion, feeling, vitality, disturbance, spiritedness, excitement, vigor, enthusiasm, turbulence, storm, sympathy.
Usage samples:
  1. He awoke from his lethargy; began to work once more in the old way; mixed among men again; grew brighter.
    "The Burial of the Guns", Thomas Nelson Page.
  2. He will no longer pine under the lethargy of ennui, that unconquerable weariness of life, more to be dreaded than death itself.
    "The Complete Poetical Works of Percy Bysshe Shelley Volume III", Percy Bysshe Shelley Edited by Thomas Hutchinson, M. A..
  3. And thus Prussia will perish, unless some powerful impetus from abroad, some dispensation of Providence, should arouse her from her lethargy, and restore her to the consciousness of her disgrace and her strength.
    "NAPOLEON AND BLUCHER", L. Muhlbach.
  4. Some four hours of this lethargy, and he became conscious, at intervals, of a sound of footsteps in his room.
    "Checkmate", Joseph Sheridan Le Fanu.