Caffeine is a drug that is used to stimulate the central nervous system, and is widely found in many plants, including tea and coffee. It is often used as a stimulant to increase alertness and decrease fatigue.
In Australia, most people have caffeine in some form everyday. Adults over 30 years old consume on average 170mg of caffeine, with that number increasing up to 250mg for adults over 70 years of age. The recommended daily limit is 400mg, which is the equivalent of about 4 cups of coffee, or 2kg of milk chocolate. However, this depends very much on the individual, as some people are particularly sensitive to it, but others can have three cups of coffee in the morning before work and be seemingly not effected.
Caffeine works by stopping the adenosine effect. As you use your brain during the day, neurons fire and release adenosine. This builds up in your body, and special receptors monitor your adenosine levels. As the day goes on, this level increases, which is why you feel tired at night.
Adenosine is the chemical that causes the onset of sleep by dilating blood vessels and slowing down brain activity. Caffeine, by binding onto the receptors, causes a withdrawal of adenosine in the postsynaptic neuron, and thereby reduces fatigue. It does this by having a very similar chemical structure to adenosine, and so your adenosine receptors can’t tell the difference between caffeine and adenosine. Basically, caffeine blocks the chemical that makes you fall asleep from doing its job.
The decrease in adenosine levels also increases the level of dopamine and epinephrine (adrenaline) in your brain. This will give you an increase in energy, and produce the “fight or flight” response. Your pupils open up, heart rate rises, breathing tubes open up, and blood flow to muscles increases. The effect of increased alertness is more prominent depending on how much caffeine is consumed. It stimulates the cardiac muscle, acts as a diuretic and increases metabolism.
Caffeine is classified as safe for human consumption by the Food and Drug Administration, so there is no need to worry. It is interesting to see how easily the brain can be tricked, and how this can have such major effects. Caffeine is basically just a copycat chemical that binds onto receptors in the brain to stop you from falling asleep.