The Collaborative International Dictionary of English v.0.48:

Insomnia \In*som"ni*a\, n. [L., fr. insomnis sleepless; pref.
   in- not + somnus sleep.]
   Lack of sleep; inability to sleep, especially when chronic;
   wakefulness; sleeplessness.
   [1913 Webster]

WordNet (r) 3.0 (2006):

insomnia
    n 1: an inability to sleep; chronic sleeplessness [ant:
         hypersomnia]

Moby Thesaurus II by Grady Ward, 1.0:

88 Moby Thesaurus words for "insomnia":
   abscess, ague, alertness, all-night vigil, anemia, ankylosis,
   anoxia, apnea, asphyxiation, asthma, ataxia, atrophy, backache,
   bleeding, blennorhea, cachexia, cachexy, chill, chills, colic,
   consciousness, constipation, convulsion, coughing, cyanosis,
   diarrhea, dizziness, dropsy, dysentery, dyspepsia, dyspnea, edema,
   emaciation, fainting, fatigue, fever, fibrillation, flux, growth,
   hemorrhage, high blood pressure, hydrops, hypertension,
   hypotension, icterus, indigestion, inflammation, insomniac,
   insomnolence, insomnolency, itching, jaundice, labored breathing,
   lidless vigil, low blood pressure, lumbago, marasmus,
   nasal discharge, nausea, necrosis, pain, paralysis, pruritus, rash,
   restlessness, rheum, sclerosis, seizure, sentience, shock,
   skin eruption, sleeplessness, sneezing, sore, spasm, stress, tabes,
   tachycardia, tension, tossing and turning, tumor, upset stomach,
   vertigo, vigil, vomiting, wake, wakefulness, wasting