Caffeine: a summary

Caffeine is probably the most commonly used chemical stimulant. More
than 90% of Americans consume caffeine on a daily basis in their
coffee, soda, tea, or chocolate. Pure caffeine is a white
bitter-tasting powder that can be extracted from coffee, tea, the
guarana berry, or from other sources.

Caffeine is a brain stimulant. It works in the brain by blocking the
natural effect of adenosine on the nerve cells, causing increased
mental activity. It also causes the body to release adrenaline, which
causes the heart to beat faster, blood pressure to rise, muscles to
tighten up, and extra sugar to be released into the bloodstream for a
burst of energy. This is why it is often used to counteract
sleepiness.

Caffeine is an addictive substance. When the effects from caffeine
wear off, the feelings of fatigue, depression, and irritability set
in. Taking more caffeine can counteract these feelings, but this leads
to a dangerous cycle. With prolonged use, it can interfere with the
brain's ability to reach deep sleep and become fully rested, which
means that the person will feel tired in the morning and be more
inclined to take even more caffeine.

credit to Braingle