Caffeine's half-life averages five to six hours but ranges from 1.5 to 9.5 hours depending on genetics, liver function, and medications. The afternoon coffee that feels fine to you may still be blocking adenosine receptors at midnight.
A 200 mg coffee at 2 PM leaves roughly 100 mg in your system at 8 PM and 50 mg at midnight — enough to fragment deep sleep even if you fall asleep on time.
L-theanine, decaf, a short walk, cold water on the face, or a 20-minute nap all produce alertness gains without the half-life problem.
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