Firstly, human blood pressure is not a fixed value like height or weight but constantly changes. Blood pressure can increase during excitement, nervousness, or exercise, and decrease after drinking, bathing, or smoking. Medical experiments have shown that a normal person's blood pressure fluctuates during the day, with the lowest levels during sleep and the highest between 8 to 10 in the morning. The normal range for systolic pressure (high pressure) varies between 20 to 40 mmHg, and for diastolic pressure (low pressure) between 10 to 20 mmHg. Even during sleep, there can be fluctuations of 5 to 10 mmHg.

Furthermore, the posture during measurement can also affect the results. Research has shown that for every 10 cm increase in the measuring position relative to the heart, the blood pressure decreases by about 8 mmHg, and vice versa.

Therefore, to ensure comparability, it's important to measure at the same time every day using the same method.