Caffeine has a half-life of approximately 6 hours and a quarter-life of 12 hours, meaning a significant amount can remain in the system long after consumption.

“On average caffeine has a half life of about 5 to six hours for some people it's quicker and for other people it's slower why is that it's based on a gene and we know the gene it's a gene that is part of a set of liver enzymes and the gene is called the cyp1 A2 Gene and there are variations in that Gene what we call polymorphisms”

“caffeine has something that we call a halflife of about 5 to 6 hours meaning that after 5 to 6 hours about 50% of that caffeine is still circulating in your bloodstream and thus your brain that means that caffeine has a quarter life of somewhere between 10 to 12 hours”

“the half-life of caffeine in blood is ~5–6 hrs. Some are genetically slow metabolizers. Factors like pregnancy, oral contraceptives, and other drugs can extend caffeine half-life to 8–10 hrs.”

“the problem with caffeine is that it actually has a half-life of about 6 hours it has a quarter life of 12 hours so in other words if you have a cup of coffee at midday a quarter of that caffeine is still in your brain at midnight”