Caffeine is found in so many different aspects of daily life. Coffee, tea, chocolate, soda, and other items contain caffeine. Caffeine can be a powerful headache trigger, and should be used with caution.
Caffeine is a stimulant, and for many people if they "artificially" inflate their energy levels with caffeine they then crash when it wears off, causing them to then crave more caffeine. It sets up a high-low cycle that demands more caffeine. The person can then get addicted to caffeine and have serious side effects when their body runs low on caffeine. These symptoms can include headaches and inability to focus.
Even if you decide to reduce your caffeine intake, it's important to be cautious about how you go about this. If your body has gotten used to the caffeine, it can react poorly to a sudden removal of that flow. This can cause a "rebound headache" which can be quite serious.
A friend of mine who was heavily addicted to caffeine intake had to stop taking the caffeine for medical reasons. She was under the supervision of a doctor - and even so she was in serious pain for a solid week during the process.
I personally used to be over the 500mg a day "maximum use" level - which is fairly easy to do if you drink coffee and/or soda. I found I was prone to shaky hands and headaches. I weaned myself off - slowly, carefully - and my health improved vastly once caffeine was out of my system. Not only that, but my body adjusted fairly quickly and soon I had more energy than I ever had before with the caffeine intake. My body didn't need caffeine to "wake me up" - it found its own natural rhythms.
If you are experiencing headaches or high-low energy cycles, look into how much caffeine you are ingesting in a given day. Make sure you count coffee, tea, soda, medications, and any other source of caffeine in your life. Then, with a doctor's help, look for ways to begin reducing it. You could easily find that your health becomes much better once you minimize the caffeine in your world!