When to take caffeine so it doesn't wreck your sleep
Caffeine doesn't clear the body instantly — its half-life (the time for blood concentration to drop by half) is roughly 5–6 hours for the average adult, though it varies between people based on genetic differences in metabolism and can range from 2 to over 10 hours. That means a cup of coffee at 4pm still leaves a meaningful amount of caffeine in the body by 10–11pm bedtime, even if someone doesn't subjectively feel alert anymore.
Caffeine interferes with falling asleep and reduces the share of deep, restorative sleep even when someone doesn't notice obvious insomnia — some studies find measurably worse sleep quality from caffeine taken 6 hours before bed, with participants not subjectively connecting it to caffeine at all.
For people who use caffeine before evening workouts to boost performance, a reasonable compromise is shifting caffeine intake earlier in the day, using a reduced dose for evening sessions, or on especially late training days, choosing sleep quality over caffeine's peak effect — particularly if quality sleep is already the weak link in recovery.