It is normal that blood pressure fluctuates during the day. Consecutive measurement will show a circadian rhythm over a 24-hour period. Usually highest reading will be observed in the morning, while lowest readings will be observed in the evening or during the night, when body is at rest. It is also important to note that blood pressure values may be affected by adrenaline, emotions, exercise, sleep patterns and digestion so make sure you pick a good time to do the screening, or otherwise your measurements will not be accurate. It is advisable to repeat the measurement up to three times in a row (with a pause of few minutes in between) to establish a correct value.
Blood pressure is assessed using two parameters -- the systolic and diastolic pressures -- which measure, respectively, the maximum pressure exerted in the arteries as the heart contracts, and the minimum pressure in those vessels between cardiac contractions. In adults, blood pressure is considered normal if the top number (systolic pressure) is between 90 and 120 and the bottom number (diastolic) is between 60 and 80.