Caffeine follows first order kinetics, which means you have a logarithmic decay in your circulating concentration. Sample graph:
Now, there is some minimal concentration that has to be circulating to have an appreciable pharmacologic effect–the 12.5 mg you’ve got floating around at 8pm likely isn’t doing a whole lot. This threshold (and your half-life) is subject to inter-individual variability.
Not every compound or drug follows first order kinetics. Most notably, ethanol is metabolized by zero order kinetics.
Pharmacokinetics (Deranged Physiology): First order, zero order and non-linear elimination kinetics | Deranged Physiology
Caffeine-specific info: Pharmacology of Caffeine - Caffeine for the Sustainment of Mental Task Performance - NCBI Bookshelf