Standard deviation of human height (within a single sex) is about 7cm. If human height changes over the course of a day by 1.5cm, assuming it linearly declines over the day, that's a stddev of about 0.6cm. You also can reasonably assume that people are normally going to be measured at the doctor/researcher's office during business hours, so it's probably more like 0.4cm. The measurement error of just standing up straight or not and reading the numbers is probably more.