In Defense of Caffeine

Have you ever felt confused about whether caffeine is good or bad for you? I certainly have, and because of this, my caffeine intake has fluctuated between total abstinence and weeks of consuming over 600 mg caffeine each day (that’s a lot). When I was doing my PhD on the relationship between nutrition and sleep, I read study after study about caffeine, and this process helped me understand how to use caffeine to get more out of life. I’ve since seen many nutrition companies maligning caffeine and, honestly, I think most of this propaganda reflects the fact they don’t read scientific publications. I’m therefore keen to clear up what we actually know about this substance. Most important, I want to share how you can use caffeine to be at your best.


Key takeaways 

  • Smart use of caffeine can improve mood, attention, reaction time, and performance in goal-directed cognitive tasks. These effects are probably strongest after poor sleep. 
  • Repeated small doses of caffeine can keep you alert and safe if you ever have to stay awake for an extended period. 
  • Used intelligently, caffeine enhances performance in practically all forms of exercise. The amount of caffeine needed to boost exercise performance is higher than the amount needed to sharpen your mind. 
  • The optimal doses of caffeine for cognition and exercise are probably slightly higher after poor sleep than well rested. 
  • Consuming too much caffeine too late in the day worsens sleep. 
  • There are huge differences between people in how they respond to caffeine, but it’s generally smart to stop consuming caffeine by about 8 h before sleep.
  • Pregnant women, nursing mothers, people with some diseases (e.g., liver diseases), and individuals taking certain medications should consume less caffeine than the rest of us. 
  • By using caffeine strategically, you can enjoy all of its upsides with minimal or no downsides.


How does caffeine affect your brain?

After you consume caffeine, almost all of it is rapidly absorbed in your stomach. Then, as it circulates through your body, caffeine can pass all your body’s membranes, including the ones in your brain. 

Perhaps the most important characteristic of caffeine is that it’s structurally similar to a chemical named adenosine, meaning that caffeine can occupy all of adenosine’s receptors in your body, temporarily blocking the actions of adenosine. Adenosine is a barometer of how long you’ve been awake and promotes sleep: The longer you’ve been awake, the greater the adenosine accumulation in your brain and the sleepier you feel. The reason for this is that adenosine inhibits the release of various alertness-promoting chemicals (neurotransmitters) in the brain, such as acetylcholine, dopamine, and noradrenaline. So, largely by inhibiting the inhibitory effects of adenosine, caffeine acutely affects brain function in the following ways: 

One thing to note is that the above research focused on single doses of caffeine. Interestingly, repeated small doses of caffeine might come into their own during extended wakefulness (e.g., when pulling an all-nighter) by minimising the negative effects of staying awake on attention and reaction time. In these circumstances, repeated low doses of caffeine also help prevent sleep attacks (i.e., falling asleep for more than 30 seconds when the goal is to stay awake), which can be critical to safety. Used wisely, caffeine can literally be lifesaving.


How does caffeine affect the rest of your body? 

Caffeine doesn’t just affect your brain, for there are adenosine receptors throughout your body, including in your cardiovascular system, respiratory system, and genitourinary system. And while caffeine primarily acts by blocking adenosine, it exerts its effects through other mechanisms too. In some tissues, for instance, caffeine weakly inhibits an enzyme named phosphodiesterase, which would otherwise have broken down a “second messenger” named cyclic adenosine monophosphate. As a result, caffeine triggers a cascade of cellular responses leading to relaxation of parts of the respiratory system (which is why caffeine is helpful for asthma) and stimulatory effects on the heart.  

Beyond the effects on cognition mentioned above, well-documented effects of caffeine include:

  • Increased heart rate. 
  • Increased blood pressure (probably only in people who don’t regularly consume caffeine). 
  • Improved airway function in asthma. 
  • Increased energy expenditure (thermogenesis). 
  • Stimulation of gastrointestinal motility… if you’ve ever had the urge to “drop the kids at the pool” after a strong coffee, this is why! 
  • Increased urination, at least in women. Caffeine doesn’t seem to affect urine production during exercise though. 

The latter point brings us seamlessly to exercise.  


Effects of caffeine intake on exercise performance 

Caffeine can reduce pain and how hard exercise feels, which we would expect to positively affect performance. Sure enough, caffeine increases output in numerous types of exercise: 

We also know that caffeine consumption changes physiology during submaximal exercise, increasing how much oxygen we consume and how much fat we burn and raising blood sugar and lactate (an important but misunderstood fuel source). 

I want to be clear that I’m not cherry-picking studies here: The studies I’ve linked to are almost all “umbrella reviews” and “meta analyses”, which rigorously and systematically review all the literature on a given subject to best determine what the entire body of evidence shows.


What about caffeine and sleep? 

Good question. 

Going by most of the studies done to date, high caffeine intakes make people take longer to nod off, reduce sleep duration, increase night-time awakenings, and worsen how people feel they slept 

Not ideal. 

We also know that if you consume too much caffeine too late in the day, you’ll delay your body’s clock and thereby shift your sleep later. 

But many of these studies have included large doses of caffeine, so what about if you regularly consume modest amounts of caffeine and don’t have any obvious sleep problems? 

In a study published this year, healthy young men who habitually consume caffeine went through 3 x 10-day conditions:  

  • Caffeine (3 × 150 mg caffeine daily, at 0.75 h, 4.25 h, and 7.9 h after waking, respectively). 
  • Withdrawal (3 × 150 mg caffeine for 8 days, then placebo for 2 days). 
  • Placebo. 

As 150 mg caffeine is roughly the amount in 2 espresso shots, caffeine intake approximated 6 espressos a day, with the last one about 8 h after waking. That’s quite a lot of caffeine. 

After 9 days in each condition, sleep was assessed using the gold-standard method.  

The results? 

Sleep was basically identical between the three conditions. The only difference was a very minor difference in patterns of brain electrical activity between placebo and the other conditions, which probably reflects acute caffeine withdrawal. 

So, if you feel your sleep is fine and you habitually consume moderate amounts of caffeine up until around lunchtime, you probably shouldn’t lose sleep over your caffeine intake! 


How to use caffeine intelligently  

The best way to use caffeine depends on many variables, not the least of which is your personal tolerance. There are huge differences between people in how they respond to caffeine, and these depend in part on variation in our genetics (specifically, genes that encode certain adenosine receptors and enzymes involved in the metabolism of caffeine), liver health (some liver diseases slow caffeine metabolism), and lifestyle (e.g., smoking seems to accelerate caffeine breakdown). Remembering this caveat, here are some recommendations if you want to leverage the advantages of caffeine: 


Table 1: Recommended caffeine intakes 



To boost cognition (when well rested) 

1 mg caffeine per kg bodyweight 1 h before the cognitive task 

To boost cognition (after sleep disruption or sleep deprivation) 

2 mg caffeine per kg bodyweight 1 h before the cognitive task 

To sustain cognition during extended wakefulness (e.g., a night shift) 

1 mg caffeine per kg bodyweight every 3 h when sleepy 

To boost performance in exercise lasting less than 5 h (when well rested) 

3 mg caffeine per kg bodyweight 1 h before exercise 

To boost performance in exercise lasting less than 5 h (after sleep disruption or sleep deprivation) 

4 mg caffeine per kg bodyweight 1 h before exercise 

To sustain performance in exercise lasting more than 5 h and/or during extended wakefulness (e.g., running through the night) 

2 mg caffeine per kg bodyweight every 3 h, beginning well into the exercise bout (limit intake to 10 mg caffeine per kg bodyweight per 24 h) 


For more on the caffeine contents of foods and drinks, check out this website. Here is the amount of caffeine in some popular foods and drinks: 


Table 2: Typical caffeine contents in commonly-consumed foods and drinks* 


Caffeine content (mg) 

Instant coffee (cup) 


Espresso (shot) 


Decaff coffee (cup) 


Black tea (cup) 


Green tea (cup) 


Decaff tea (cup) 


Yerba maté tea (cup) 


Matcha tea (cup) 


70%-cocoa dark chocolate (20g) 


*Note that there is substantial variation within these items, depending on variables such as the type of coffee beans used and how long tea is brewed for.


Now, the recommendations in Table 1 are based on my interpretations of the research at large, and I want to be clear that there are nuances to consider. For example, the dose of caffeine needed to enhance executive function is higher than the amount required to boost attention, but I’ve lumped these into “cognition”.  

Another thing to note is that my recommendations are probably on the low end: In many instances, there’s a dose-response relationship between caffeine intake and performance, with slightly higher doses having the biggest short-term effects. However, high doses (over 6 mg per kg or so) increase risk of side-effects such as jitteriness and poor sleep the following night, and I can’t think of any instances in which I’d recommend a dose higher than 10 mg caffeine per kg bodyweight per 24 h. 

Please also realise that if you want to sleep well, when you consume caffeine relative to sleep matters. As a starting point, I recommend you stop consuming caffeine at least 8 h before you plan to sleep. If you must have coffee or tea after this time, pick decaff. Finally, the more caffeine you consume, you earlier you should stop relative to sleep.  


Who should be wary of consuming lots of caffeine? 

I also want to be clear that some of us should be a bit wary of caffeine: 

  • Pregnancy dramatically slows caffeine metabolism, and this effect is strongest in the last trimester of pregnancy. Modest doses seem to be safe though, and the American College of Obstetricians and Gynecologists recommends limiting intake to 200 mg per day while pregnant. 
  • Nursing women pass on caffeine to infants via breastmilk, and children younger than 9 months or so metabolise caffeine very slowly. 
  • People with certain diseases should not go overboard with caffeine. These diseases include anxiety, cardiovascular disease, peptic ulcer disease, gastroesophageal reflux disease, liver disease, kidney disease, and seizures.  
  • Consuming caffeine with some other drugs can pose problems (e.g., alcohol slows caffeine metabolism dramatically). 


How not to use caffeine 

I strongly suggest that you use caffeine strategically. If you consume large amounts of it every day, you’ll develop tolerance to some of its effects and start feeling like you need it just to feel human. And if for some reason you then can’t consume it one day, you’ll probably experience withdrawal effects such as fatigue and headaches (you can prevent these by tapering off caffeine gradually).  

So, a better approach is to keep your habitual intake modest, saving higher doses for when you need them. Using myself as an example, if I keep my habitual intake below about 2 mg caffeine per kg bodyweight per day, I don’t feel dependent on it to function and don’t get any withdrawal effects on days when I don’t have it.

Another mistake I’ve seen a lot of people make is not realising that certain foods and drinks are caffeine rich. For example, some of us eat dark chocolate late in the day and thereby inadvertently consume lots of caffeine shortly before bed. My favourite dark chocolate contains about 150 mg caffeine in half a bar (45 g), roughly the amount of caffeine in a double espresso! 


Time for a brew  

Hopefully this article has persuaded you that caffeine can be an exceptionally helpful substance. And I haven’t even touched on the fact that most of the plants that are naturally high in caffeine are also loaded with other health-promoting substances (e.g., chlorogenic acid in coffee, flavanols in cocoa, and L-theanine in tea)… more on this later. 

So, if you’ve been eschewing caffeine because you mistakenly think it’s bad for you, it’s time to wake up and smell the coffee: Learn how to use caffeine intelligently, and you’ll gain an advantage over your peers. 

I’m off for a brew ;-) 




Disclaimer: This article may have been produced under the influence of caffeine.