Coffee and tea contain caffeine that can help you be awake and alert.  8 out 0f 10 Americans consume caffeine in some form.  

So how does caffeine work?  It blocks the receptors in the brain for a chemical called adenosine that is naturally made in your body.  It is adenosine that will make you feel tired at the end of the day.  However, if you take in a lot of caffeine, the body will make more adenosine to override the receptor block.  This means you will need to intake more and more caffeine to get the same response.  

Adenosine is also responsible for the headaches when frequent caffeine intake is  abruptly stopped.  Other effects of caffeine withdrawal are: drowsiness, irritability, nausea and difficulty concentrating.

There may be other issues with caffeine.  It is also a very mild diuretic that may make you urinate more often.  It may increase the acid production in your stomach causing heartburn, and may cause high blood pressure.   Drinking very hot beverages may increase your risk of esophageal cancer.  Caffeine may be a trigger for migraines in certain genetically susceptible people. 

Too much caffeine may cause a feeling of nausea, anxiety or a racing heart.

Different people break down caffeine at different rates depend on their genetics.  People who breakdown caffeine more slowly may be at an increased risk of having side effects from caffeine. 

Women who are pregnant or breastfeeding, children, people who have insomnia or other sleep problems, anxiety, heartburn, fast or irregular heartbeat, or have high blood pressure should not have a lot of caffeine

Caffeine has been linked to benefits such as decrease in heart disease, heart failure and stroke, decrease in type 2 diabetes, decrease in Alzheimers and Parkinsons disease, decrease in colon, uterine and liver cancer as well as a decrease in cirrhosis.

Here are the amounts of caffeine in common beverages.

An 8-ounce cup of coffee: 95-200 mg

A 12-ounce can of cola: 35-45 mg

An 8-ounce energy drink: 70-100 mg

An 8-ounce cup of tea: 14-60 mg

The FDA recommends moderate use of caffeine, 2-3 cups of coffee or about 400mg a day.  It should be noted that there may be very high doses of caffeine in pills or powder and this if taken in excess may be fatal.  These products should be avoided.  Excessive alcohol intake is also associated with drinking alcoholic caffeinated drinks.  This should be avoided as well.