willywonka
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Drowsiness is caused by the binding of andenosine to andenosine receptors in the brain that slows down nerve cell activity. To a nerve cell, caffeine looks like adenosine. Caffeine therefore binds to the adenosine receptor. However, caffeine doesn't slow down the cell's activity like adenosine would. So the cell cannot "see" adenosine anymore because caffeine is taking up all the receptors adenosine binds to. So instead of slowing down because of the adenosine level, the cells speed up. You can see that caffeine also causes the brain's blood vessels to constrict, because it blocks adenosine's ability to open them up. This effect is why some headache medicines like Anacin contain caffeine -- if you have a vascular headache, the caffeine will close down the blood vessels and relieve it. Caffeine also increases dopamine levels in the same way that amphetamines do. So you can see why your body might like caffeine in the short term, especially if you are low on sleep and need to remain active. Caffeine blocks adenosine reception so you feel alert. This is the reason most Americans consume caffeine every day
