Blood pressure varies during a 24 hour day in a regular manner, i.e. a circadian rhythm. Systolic and diastolic pressure are high during day-light hours, and fall at night. These changes occur to some degree even if the individual remains active at night, or if he sleeps during the day. Clearly, these changes must be allowed for when comparing blood pressure measurements made during the day and at night, otherwise erroneous conclusions might be drawn about a persons health.
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