How does caffeine affect blood vessels?

Caffeine affects blood vessels both as a vasodilator and a vasoconstrictor. It blocks the action of adenosine, which is a vasodilator, thus leading to vasoconstriction. This can help relieve headaches caused by excessive vasodilation. However, caffeine has the ability to impinge on the nitric oxide pathway, which can also lead to vasodilation. The effects of caffeine on blood vessels can vary depending on factors such as the time of day and the individual's adenosine levels. If adenosine levels are low, such as after a good night's sleep, caffeine may lead to more vasodilation. Conversely, higher adenosine levels, such as from lack of sleep or it being later in the day, can cause more vasoconstriction when caffeine is ingested 1.