Fluctuations in blood pressure within a given day are quite common. The blood pressure is influenced by a combination of hormones, nerve signals, and the amount of fluid in the bloodstream. A large meal might contribute to your blood pressure because of the salt that it contains when salt is absorbed into the bloodstream, it pulls water into the blood from other tissues and in this way adds to your blood pressure. If you are a diabetic, an increase in blood sugar following your meals could have a similar temporary effect, again pulling water into the blood. Evening meals commonly include coffee; it is possible for a blood pressure increase to come from caffeine, which can cause temporary tightening of your arteries. Finally, you should consider the possibility that your blood pressure increase might have nothing to do with the evening meal, but may indicate that a morning medication for blood pressure is wearing off.
In some cases, when the blood pressure is widely varied, it is helpful to monitor the blood pressure at close intervals through a full day so that the average blood pressure can be determined. This can be done with a device known as an ambulatory blood pressure monitor. Blood pressure is typically considered to be in an acceptable range if its 24-hour average is below 135/85, or if its daytime average is less than 140/90. The difference in these numbers reflects the fact that blood pressure in most people lowers naturally during the night.
