TL;DR When’s the ideal time to drink coffee? How many minutes before caffeine kicks in Those are valid questions. In fact, caffeine takes some time before it goes into full gear. And there are a number of factors that make caffeine stay longer in your system.
If you’re one of the many coffee-drinkers who regularly drink a cup of joe for the morning jolt, then you must’ve wondered at one point:
When’s the ideal time to drink coffee? How many minutes before caffeine kicks in?
These are valid questions. In fact, caffeine’s nothing like Popeye’s spinach that works the moment he pops that can of power.
Caffeine takes some time before it goes into full gear.
So the million-dollar question is - how long does it take for caffeine to work?
That’s exactly what we’re going to find out in this article.
You might feel drowsy and incapable of functioning at a high level until the caffeine hits your bloodstream. You may go from 0 to 60 in about 10 minutes, but you hit top speed after about 45 minutes, when 99 percent of the caffeine from your coffee, tea or energy drink has entered the bloodstream fully. Then it takes many hours for the caffeine to dissipate.
These findings were reached by researchers from the University of Barcelona in a study published in December 2008 in the journal Progress in Neuro-Psychopharmacology and Biological Psychiatry. 
The 10-minute finding contradicted previous studies that showed alertness does not increase until after 30 or 45 minutes, according to an article in Science Daily. 
To measure caffeine’s effects on the human body, the researchers analyzed 668 university students, 238 of them male. Their average age was 22. They sampled the levels of caffeine in their blood at 10, 20 and 30 minutes. They took the measurements two times during the day after the students drank coffee, at midday and in the late afternoon, to serve as a control if there were possible differences caused by the time of day.
“Forty-five minutes is the time needed for maximum caffeine concentration to be reached in the blood, but levels reach half this concentration after just a few minutes,” Science Daily reports.
To cut the long story short, the magic number based on these reports appears to be 10-45 minutes. During this period, you’ll feel caffeine’s effects in full steam, like an energy boost, optimal mental performance, enhanced sports performance, a happy mood.
Or if you have too much than your body can usually take, you’ll also get the adverse effects - anxiety, depression, jitters.
Afterward, the liver metabolizes caffeine and out it goes from your body.
So if you have an important presentation or an exam, then better have that first pop of caffeine mints or a cup of java 10-45 minutes prior!
Now that we know how long the effects of caffeine are felt after that first sip (or mint pop), the next question is how long caffeine actually stays in our system.
The University of Nottingham School of Health Sciences defines half-life as: 
the duration of action of a drug [and the] period of time required for the concentration or amount of drug in the body to be reduced by one-half.
We usually consider the half-life of a drug in relation to the amount of the drug in plasma. A drug’s plasma half-life depends on how quickly the drug is eliminated from the plasma.
While half of the total caffeine dosage consumed get cleared from the body in 5 hours for most people, it normally takes 8.25 hours to 2.18 days to completely rid it of the stimulant.
How long caffeine stays in your body depends on several factors.
For adults, the half-life is usually 5 hours. For babies and kids? It’s 80 hours. Big difference, right?
Here’s something interesting - the caffeine metabolism of senior individuals aged 65 and above of 1.0 ml/min is 33% lower than most adults’ 1.5 ml/min.
This may sound like common sense and not that hard to imagine.
The bigger, taller and heavier an individual is, the faster the rate of caffeine metabolism is. Having more body fat can help make the metabolism and clearance of caffeine from your body faster.
The effects of caffeine and the body’s ability to metabolize it may differ based on genetics. Some people are able to process caffeine 40 times faster than others. That applies to the rate by which the body goes through caffeine clearance. [9, 10]
If you drink coffee or tea on an empty stomach, you’re more likely to feel its effects faster than when you’re eating a bagel with it. If you eat fiber-rich food, then it’ll take some time for caffeine to go full blast in your system.
And because of the impact of food on caffeine absorption, the same may be gleaned with how rapidly the body can clear itself off caffeine with and without food.
People with a healthy liver and kidneys tend to metabolize caffeine at a normal rate, with a half-life of 5 hours. But those with impaired liver and kidneys will find it harder and slower to break down the caffeine.
According to studies, those with liver dysfunction will have a caffeine half-life anywhere between 60 to 168 hours - a massive spike from the usual 5 hours! 
Kidney impairments may also lead to slower metabolism but not to the same extent as with unhealthy liver.
Men obtain more of the stimulating effects of caffeine than women. An article by Science Daily reports: 
“Although both the men and women saw an improvement in their activity levels with the coffee, which increased in later measurements, we observed a greater impact among the males.”
One reason why this is the case? Some women take oral contraceptives, which can slow down caffeine metabolism by an extended 3 hours. 
Some drugs can also accelerate the breakdown of caffeine and clearance from the body. Others can keep caffeine a little longer than usual.
Because caffeine is broken down in the liver with the help of CYP1A2 isoenzymes, substances that interact with these also affect the rate of metabolism and clearance.
Those who take drugs that induce CYP1A2 will experience faster clearance. These inducers include Insulin, Modafinil, Nafcillin, beta-Naphthoflavone, and Omeprazole.
Read this article to find out whether you can combine caffeine with certain medications.
On a similar vein, smokers tend to have a quicker clearance rate than non-smokers with 155 ml/kg/hr compared to non-smokers at 94 ml/kg/hr. In fact, smokers are reported to have faster caffeine half-life with 2.5 hours!
The more you have caffeine, the slower you get to metabolize and excrete it.
Just by the sheer amount of caffeine in your body will make it take a little longer to complete the metabolism.
According to Mental Health Daily: 
A frequent coffee drinker may consume several cups throughout the day, resulting in ~300 mg of ingested caffeine. After the third cup of coffee, the body will not have fully metabolized nor excreted the caffeine content within the first two cups.
As a result, a greater total amount of caffeine metabolites (i.e. paraxanthine) will have accumulated throughout a frequent user’s system. Greater accumulation of caffeine may result in a higher degree of reabsorption prior to elimination, ultimately prolonging clearance times. Furthermore, since the frequent coffee drinker may ingest their final (third) cup of coffee later in the day (e.g. afternoon), it won’t be metabolized nor eliminated as quickly as someone who had just one cup in the morning.
Another easy explanation here -
The less caffeine you take, the quicker your body breaks it down.
According to the same article on Mental Health Daily: 
An extremely low dose of caffeine would be within the 10 mg to 20 mg range. This range is just enough to feel its effects, and the body shouldn’t have any difficulty efficiently metabolizing such a low dose. Most people consume around 200 mg of caffeine per day, resulting in slower metabolism compared to an individual ingesting a lower dose such as 10 mg.
A heavy caffeine consumer may ingest over 400 mg per day (equivalent to 4 cups of coffee). At this point, enzymes in the liver may be overtaxed and more caffeine (and its metabolites) may accumulate within the body. This accumulation may prevent efficient clearance and result in reabsorption, prolonging excretion times relative to dosage consumed.
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Some research has suggested that caffeine may stimulate thermogenesis - a scientific name for the way your body generates heat and energy from the calories in your food; but nutrition experts say that this effect probably isn't enough to produce significant weight-loss. Caffeine may also reduce your desire to eat for a brief time, but again, there's no good evidence over the long-term that this effect leads to weight-loss. To date, no conclusive clinical studies have been done to determine the long-term effect of caffeine on weight loss, and the smaller studies that have been done show a lot of variability in the outcomes.
Want to hear something shocking?
Having your caffeine fix first thing in the morning will NOT perk you up.
But the good news is, you no longer need to make that sluggish early morning trip to the coffee-maker daily, nor join that long rush hour queue in your go-to café.
If you’re wondering whether we’re pulling some sick April Fool’s joke in the middle of August, there’s actually scientific evidence to all of this.
If you’re trying to lose weight (or at least not gain a few extra pounds), then the best thing to do is eat healthy and go to the gym more religiously, right?
But if you’ve been going at it for a while now and haven’t been seeing much progress, then you may want to look into something else.
Like your coffee consumption.
Now you may ask: what does an innocent cup of joe have to do with weight gain?
Let me tell you.
It’s not as innocent as it seem.
That cup of coffee you buy on your way to work? It may be sneaking in a few extra calories (more than you’d like and expect). And if you buy more than one cup a day, you may be racking up a few calories from a “dessert” that disguises itself as your go-to caffeine fix.