The field sobriety test has no scientific basis for being admissible evidence in a DUI trial, but prosecutors in court often argue that they are the sole indicators of whether someone was under the influence. They will try to have a jury give greater weight to a driver’s performance on those tests than they likely deserve.  There is an inherently unreliable aspect of field sobriety tests that a defense attorney may need to explain to a jury to properly defend against any conclusions they are the most important indicators of impairment.
A person is allowed to refuse a field sobriety test at any time during a traffic stop.  If they are tired, older, or have certain physical limitations, they should seriously consider declining the opportunity to take field sobriety tests, as there is the potential for a harmful false negative.