Caffeine stimulates glands
Why does caffeine keep you awake? Just a few minutes after you have something to eat or drink with caffeine in it, the organs and tissues in your body get a little more pep. That is because the chemistry in your cells changes.
The adrenal glands, which produce chemicals that speed things up, kick extra amounts into your system when you have caffeine.
The brain cells are among those cells that act faster. This keeps you from feeling drowsy and speeds up your reactions. This is a problem, however, if you are trying to fall asleep. The next day you may have what is known as a caffeine hangover.
In addition, the heart muscle works harder, making the blood speed up in its round-trip through your body.