Caffeine Timing: The Half-Life Arithmetic

The arithmetic of caffeine's half-life is straightforward and almost nobody does it.

The arithmetic

Caffeine's half-life averages around five to six hours in healthy adults, with wide variation. That means a dose consumed at four in the afternoon still has roughly half its amount present at ten in the evening and a quarter at four in the morning. Studies giving caffeine at fixed intervals before bed have found measurable disruption to sleep even when consumed six hours prior, including in participants who reported no subjective effect.

What it does to sleep

Caffeine reduces total sleep time, increases sleep onset latency, and reduces slow-wave sleep specifically. That last effect matters because slow-wave sleep is where much of the restorative and consolidation work happens - so caffeine can reduce sleep quality even when duration looks acceptable. It also creates a self-reinforcing cycle: poorer sleep produces more daytime tiredness, producing more caffeine intake, producing poorer sleep.

Reducing without withdrawal

Withdrawal is a recognised syndrome with headache, fatigue and low mood, typically beginning twelve to twenty-four hours after the last dose. Tapering avoids most of it: reduce by roughly a quarter every few days rather than stopping abruptly. Shifting the cut-off time earlier is often more valuable than reducing total amount - many people do well keeping their morning intake and eliminating everything after midday. Decaf still contains a small amount, which occasionally matters for very sensitive people.

If you are considering Cognizil

Read the Cognizil ingredients page before ordering and check the guarantee terms. If you take any prescription medicine — including hormonal contraception — show the label to a pharmacist first, because the St. John’s Wort content genuinely matters. Our buyer’s checklist sets out twelve checks worth running.