It's easy to consume too much caffeine when you drink it using pop such as Pepsi Zero. (Called Pepsi Max in some regions) 

In a 710 ml bottle you find a grocery store of Pepsi Zero there is 138mg of caffeine in one bottle. If you buy these in a 6 pack it's easy to consume more caffeine than you intended. 

If you have a high tolerance to caffeine and arent sensitive to it this is fine. Though caffeine is an addictive substance, mostly psychologically because it activates dopamine and blocks adenosine receptors. 

These are the neurotransmitters that signal to your brain that it's time to sleep which is the precursor to melatonin which your brain fires off at night. Caffeine hijacks this.  Sleep is important to memory and learning so you should only consume caffeine in the morning until the late afternoon and never in the evening or night.