The Collaborative International Dictionary of English v.0.48:

Sleepless \Sleep"less\, a.
   1. Having no sleep; wakeful.
      [1913 Webster]

   2. Having no rest; perpetually agitated. "Biscay's sleepless
      bay." --Byron.
      [1913 Webster] -- Sleep"less*ly, adv. --
      Sleep"less*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sleeplessness
    n 1: a temporary state in which you are unable (or unwilling) to
         sleep; "accept your wakefulness and sleep in its own
         contrary way is more likely to come" [syn: wakefulness,
         sleeplessness] [ant: drowsiness, sleepiness,
         somnolence]

Moby Thesaurus II by Grady Ward, 1.0:

29 Moby Thesaurus words for "sleeplessness":
   acuity, acuteness, agility, alertness, all-night vigil, attention,
   attentiveness, brightness, consciousness, insomnia, insomniac,
   insomnolence, insomnolency, keenness, lidless vigil, nimbleness,
   promptitude, promptness, punctuality, quickness, readiness,
   restlessness, sentience, sharpness, smartness, tossing and turning,
   vigil, wake, wakefulness