Final answer:
The average blood pressure drops by 15-20 mmHg at night due to the body's natural circadian rhythm and a decrease in sympathetic nervous activity. This nocturnal dipping is a necessary rest period for the cardiovascular system.
Step-by-step explanation:
The average blood pressure typically drops at night as the body enters a state of rest. This phenomenon is known as nocturnal dipping.
B) 15-20 mmHg.
While sleeping, the sympathetic nervous system, which is responsible for the body's fight-or-flight response, becomes less active, and the parasympathetic nervous system takes over. This shift leads to a decrease in heart rate, dilatation of blood vessels, and thus a lower systemic vascular resistance, which together contribute to the decrease in blood pressure.
The drop in blood pressure is a natural part of the circadian rhythm, allowing the cardiovascular system a period of rest. Blood pressure that does not adequately dip during the night is often referred to as 'non-dipping' and can be indicative of potential cardiovascular issues. It is important to note that an excessive drop in nocturnal blood pressure can also have adverse effects, potentially leading to what is known as 'nocturnal hypotension.'