Astress test was found to actually measure sleep deprivation. the stress test would lack:

I believe the answer is: Reliability

Reliability in a test could only be achieved if the result of the test would typically resulted in the same measurement if being taken by different people. The effect of sleep deprivation would be drastically different depending on the physical and genetic composition of the subject, so the test would not be reliable.

answer; eliminate parking problems in cities, because a car took up only half as much space as a horse and buggy;