From The Collaborative International Dictionary of English v.0.48 [gcide]:
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]
From WordNet (r) 3.0 (2006) [wn]:
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]