We present precise, ~1%, r-band relative photometry of the unusual solar system object (90377) Sedna. Our data consist of 143 data points taken over eight nights in 2004 October and 2005 January 2005. The rms variability over the longest contiguous stretch of five nights of data spanning 9 days is only ~1.3%. This subset of data alone constrains the amplitude of any long-period variations with period P to be A 1%(P/20 days)². Over the course of any given ~5 hr segment, the data exhibit significant linear trends not seen in a comparison star of similar magnitude, and in a few cases these segments show clear evidence of curvature at the level of a few millimagnitudes per hour squared. These properties imply that the rotation period of Sedna is O(10 hr); it cannot be 5 hr nor can it be 10 days, unless the intrinsic light curve has significant and comparable power on multiple timescales, which is unlikely. A sinusoidal fit yields a period of P = 10.273 ± 0.002 hr and a semiamplitude of A = 1.1% ± 0.1%. There are additional acceptable fits with flanking periods separated by ~3 minutes as well as another class of fits with P ~ 18 hr, although these later fits appear less viable based on visual inspection. Our results indicate that the period of Sedna is likely consistent with typical rotation periods of solar system objects, thus obviating the need for a massive companion to slow its rotation.