Many people develop quite a tolerance for caffeine. This means that their body gets used to having caffeine every single day. Over time, they must keep increasing their caffeine intake to achieve the desired effects - alertness and the ability to concentrate. Caffeine addiction is more likely to occur in someone that’s also suffering from a co-occurring mental health disorder (as with many addictions), such as insomnia or an eating disorder, and this can make it harder for them to stop.