Blood pressure is a means of measuring heart health. A normal blood pressure is 120/80 mmHg. Blood pressure levels are influenced by many external factors, not simply the state of the body (rest/moving); therefore, blood pressure fluctuates throughout the day.
The most important aspect that controls blood pressure is the sympathetic nervous system. The higher the sympathetic stimulation, the greater the rise in blood pressure. The most important hormone or chemical that mediates these fluctuations is primarily adrenaline. Excess production of adrenaline causes sympathetic stimulation, which leads to a fluctuation in blood pressure.
It follows the circadian rhythm. Blood pressure is normal during some part of the day and will rise during some part of the day. During early morning, as one gets up from sleep, the blood pressure will be low, because the body will be in a state of rest. Slowly, it will start rising and then reach its peak during mid-afternoon as the activity increases. There is a difference of 10-15 mmHg in systolic blood pressure and 5-10 mmHg of diastolic blood pressure.
Conditions like pregnancy, heart conditions, dehydration, and allergies will also raise blood pressure.