Caffeine has a metabolic half life of 6-12 hours. This means that after a 24 hour period, there could be 1/4 of the original caffeine amount you drank in your system. If you drink the same amount of caffeine again at that point, now after a 24 hour period you ‘ll have up to 1/4 of that 1.25 amount in your system. If you consume caffeine daily, this can lead to an accumulation of caffeine that your body adjusts to always being there, becoming the new baseline normal. This would feel fine until you stop, at which point the caffeine your body expects to be there is gone, and it needs to take time readjusting to that absence. That leads to withdrawal symptoms.