This description of the One Leg Stand test posted by California DUI Lawyer.

Studies show that closing the eyes make all of the balance and coordination tests much more difficult for sober individuals to perform. Peripheral vision plays a particularly important role in maintaining balance. By having a person close their eyes, the officer takes away their visual frame of reference necessary in order to assess the level of impairment with any degree of reliability. Nowhere in any of the SFST is a person required to close their eyes while performing any part of the tests. A test may be further compromised by placing the defendant in an imbalanced position by having them tilt their head back or bend at the waist. During over a hundred controlled drinking exercises administered or assisted by me, I have seen the vast majority of volunteers have difficulty with these type of Sway tests even when completely sober. 

Sway is expected due to the nature of this type of test. Some sway will be observed regardless, due to the person being a living breathing being. This is demonstrated not only in the Final Report of March 1981 entitled "Development And Field Test Of Psychophysical Tests For DWI Arrest" which was prepared for the U.S. D.O.T. National Highway Traffic Safety Administration" (NHTSA) regarding studies listed in same, but also in the "Administration of the Aerospace Physiological Training Program" which is used at the U.S. Air Force School of Aerospace Medicine.