The Collaborative International Dictionary of English v.0.48:

Sunup \Sun"up`\, n.
   Sunrise. [Local, U.S.]
   [1913 Webster]

         Such a horse as that might get over a good deal of
         ground atwixt sunup and sundown.         --Cooper.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sunup
    n 1: the first light of day; "we got up before dawn"; "they
         talked until morning" [syn: dawn, dawning, morning,
         aurora, first light, daybreak, break of day, break
         of the day, dayspring, sunrise, sunup, cockcrow]
         [ant: sundown, sunset]

Moby Thesaurus II by Grady Ward, 1.0:

21 Moby Thesaurus words for "sunup":
   aurora, break of day, brightening, chanticleer, cockcrow,
   cocklight, crack of dawn, dawn, dawning, day-peep, daybreak,
   daylight, dayspring, first brightening, light, morn, morning,
   peep of day, prime, sunrise, vestibule of Day