The Science of Half-Life
Many people believe caffeine leaves their system as soon as the "buzz" wears off. In reality, caffeine has a half-life of approximately 5 hours. This means that 5 hours after you drink a coffee, 50% of the caffeine is still active in your blood.
If you drink a large coffee (200mg) at 4:00 PM, you will still have 100mg (equivalent to a fresh cup of coffee) flowing through your veins at 9:00 PM.
How Caffeine Blocks Sleep
Caffeine works by blocking adenosine receptors in the brain. Adenosine is a chemical that builds up throughout the day to make you feel tired (sleep pressure). When caffeine is present, your brain cannot detect the adenosine, so you feel alert.
However, even small amounts of residual caffeine at bedtime can reduce Deep Sleep (SWS) by up to 20%, leaving you feeling groggy the next morning despite sleeping for 8 hours.
Frequently Asked Questions
Can drinking water flush out caffeine? ▼
No. Caffeine is metabolized by the liver (specifically the CYP1A2 enzyme). Drinking water helps with hydration, but it does not speed up the liver's metabolic process. Only time can clear it.
Why does caffeine affect me more than others? ▼
Genetics play a huge role. Some people have a variant of the CYP1A2 gene that makes them "slow metabolizers," meaning caffeine stays in their system much longer (up to 10 hours), causing anxiety and insomnia.