If left over the span of a day, would a cup of coffee or an energy drink lose its caffeine content?
I’m an analytical chemist in pharma. In my lab, we use caffeine as a standard specifically because of how stable it is. A solution of caffeine in water is literally stable in refrigerated conditions for literally over a year.
Caffeine in its pure form is a white solid like salt, it would be easier for the drink to evaporate and the caffeine to go with it than the caffeine to go away. In the case of caffeine, like almost any chemical substance, there will be a moment when your body will create resistance, so you will have to ingest a greater volume of drink to have the same effect, the same is true for medicines (antibiotics is even worse, select and leave only the stronger bacteria to reproduce), in case I don’t know if you’re after caffeine as a stimulant but if so try adding it to coffee or tea (I can’t drink energy drinks due to medical problems) cinnamon, ginger or even pepper, chili pepper they all have an effect stimulants