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):

sleepless
    adj 1: experiencing or accompanied by sleeplessness; "insomniac
           old people"; "insomniac nights"; "lay sleepless all
           night"; "twenty watchful, weary, tedious nights"-
           Shakespeare [syn: insomniac, sleepless, watchful]
    2: always watchful; "to an eye like mine, a lidless watcher of
       the public weal"- Alfred Tennyson [syn: lidless,
       sleepless]

Moby Thesaurus II by Grady Ward, 1.0:

123 Moby Thesaurus words for "sleepless":
   Argus-eyed, agile, alert, alive, all ears, all eyes, ardent,
   assiduous, attentive, aux aguets, awake, bright, constant,
   continuing, custodial, diligent, disturbed, dogged, eagle-eyed,
   enduring, energetic, faithful, fervent, guarded, hard, hardworking,
   hawk-eyed, immutable, inalterable, indefatigable, indomitable,
   industrious, insistent, insomniac, insomnious, invincible, keen,
   keen-eyed, laborious, lasting, lidless, loyal, never idle,
   never-tiring, nimble, observant, obstinate, on guard, on the,
   on the alert, on the ball, on the job, on the lookout,
   on the watch, open-eyed, patient, patient as Job, permanent,
   perseverant, persevering, persistent, persisting, pertinacious,
   plodding, plugging, preoccupied, prompt, prudent, qui vive, quick,
   rapt, ready, relentless, resolute, restless, sedulous, sharp,
   sharp-eyed, single-minded, slogging, slumberless, smart, stable,
   steadfast, steady, strenuous, stubborn, tenacious, tireless,
   unabating, unblinking, unconquerable, undaunted, undiscouraged,
   undrooping, unfailing, unfaltering, unflagging, unflinching,
   unintermitting, uninterrupted, unnodding, unrelaxing, unrelenting,
   unremitting, unsleeping, unsparing, unswerving, untiring,
   unwavering, unwearied, unwearying, unwinking, utterly attentive,
   vehement, vigilant, wakeful, wary, watchful, weariless, wide-awake,
   with open eyes, zealous