Caffeine has a half-life of roughly five hours, which is why the 4pm espresso is still in your system at midnight. Log what you drank and when, and find out how much will still be circulating at bedtime.
Caffeine is eliminated roughly exponentially, with a half-life of 3–7 hours depending on the person. This tool models that decay from your last drink to bedtime: remaining = dose × 0.5^(hours ÷ half-life). Typical doses per serving are approximations. About 100 mg still circulating at bedtime is enough to notice.