Many people drink coffee, and rely on it, on a daily basis. A large number of those who consume caffeine from coffee do so to increase their energy and improve their focus.
Caffeine is the most widely consumed stimulant in the world. That being said, it does not affect everyone in the same way.
Some people, for example, can drink multiple cups during the day and experience few effects. Others may experience adverse effects after drinking a single cup of coffee.
According to the Food and Drug Administration (FDA), an average 8 ounce (oz) cup of coffee contains around 80–100 milligrams (mg) of caffeine.
In this article, we look at why coffee makes some people feel tired. We also discuss other potential effects of coffee and daily intake recommendations.
Coffee itself does not make people tired, but the caffeine in coffee and its effects on the body can sometimes cause tiredness.
Some reasons that drinking a cup of coffee might make someone feel tired include the fact that:
Caffeine blocks adenosine receptors in the brain
Share on PinterestThe chemicals in coffee can affect people differently.
Adenosine is a brain chemical that affects the sleep-wake cycle. Adenosine levels increase during waking hours and decrease during sleep.
Normally, adenosine molecules bind to special receptors in the brain, which slows down brain activity in preparation for sleep. However, caffeine prevents this from happening by binding to adenosine receptors.
The body rapidly absorbs caffeine, so people may feel its effects within minutes. In fact, the body absorbs 99% of caffeine within 45 minutes of consuming it. Once the body fully metabolizes the caffeine, its effects will wear off.
The length of time caffeine stays in the body varies from person to person. Although it blocks adenosine receptors, it does not affect the production of new adenosine molecules.
When caffeine wears off, adenosine molecules can bind to their receptors, which can cause sleepiness.
Some people build a tolerance to caffeine
People who regularly consume coffee and other caffeinated beverages can develop a tolerance to it. Since caffeine blocks adenosine receptors, the body produces more adenosine receptors to counteract the effects of frequent caffeine consumption.
In one small study, researchers examined the effects of continuous caffeine consumption on the cycling performance of 11 physically active adults.
At the start of the study, the participants had higher heart rates and exerted greater cycling power after drinking caffeine. After 15 days, however, the effects of the caffeine began to decrease.
Given these results, it appears that people who regularly consume caffeine may build up a tolerance to its stimulant effects.
However, other research has suggested that continuous exposure to caffeine does not affect how the body absorbs or metabolizes it.
Caffeine increases blood sugar levels
Coffee’s effects on blood sugar levels remains a point of controversy for researchers.
Many human and animal studies suggest that compounds inside coffee may improve glucose metabolism and reduce the risk of type 2 diabetes. However, the adverse effects of caffeine may negate the beneficial effects of drinking coffee.
According to a 2016 meta-analysis, caffeine can increase blood sugar levels by temporarily reducing insulin sensitivity.
In a small study, researchers observed that consuming 100 mg of caffeine adversely affected glucose metabolism in 10 healthy males with excess weight.
These findings suggest that caffeine, not coffee itself, can impact glucose metabolism, which can raise blood sugar levels.
People with high blood sugar may experience headaches, fatigue, trouble concentrating, increased thirst, or frequent urination until their blood sugar levels return to normal.
Coffee can affect more than a person’s energy levels. The sections below discuss a few possible effects of drinking coffee.
Insomnia
Share on PinterestDrinking coffee close to bedtime may cause insomnia.
People who drink coffee before going to bed may have trouble falling asleep.
The authors of a 2013 study recommend that people stop drinking coffee at least 6 hours before they plan to fall asleep.
Also, according to one 2016 study, researchers reported an association between higher caffeine intake and more severe insomnia in 234 middle school students in South Korea.
Anxiety
The caffeine in coffee may have adverse effects on people’s mental health. For example, high doses of caffeine (above 400 mg) can cause jitteriness and nervousness.
In people with panic disorders and depression, high doses of caffeine can trigger anxiety-related symptoms.
Cardiovascular effects
There may be temporary increases in heart rate and blood pressure after a person drinks caffeinated coffee.
However, current research suggests that mild-to-moderate coffee consumption may protect heart health.
In a 2017 study involving 557 individuals in Brazil, researchers found evidence to suggest that consuming one to three cups of coffee per day may reduce risk factors for cardiovascular disease in a population.
However, the findings of a 2019 study suggest that drinking more than six cups of coffee per day could increase cardiovascular risk.
Cancer prevention
The authors of a 2017 review article found evidence to suggest a potential association between increased coffee consumption and a reduced risk of hepatocellular carcinoma. This is a type of liver cancer.
Also, research from 2019 found that coffee compounds — including caffeine, trigonelline, and chlorogenic acid — protect against gastrointestinal and liver cancer progression.
However, more research is necessary to confirm the links between drinking coffee and cancer risk.
Other effects of drinking coffee
Other potential effects of drinking coffee include:
- increased alertness
- jitteriness or restlessness
- dizziness
- headaches
- dehydration
- stomach pain
- frequent urination
If drinking a cup of coffee makes a person feel tired, the effects of caffeine may be responsible.
Caffeine increases alertness by interfering with certain chemical processes in the brain that regulate the sleep-wake cycle. However, once the body completely metabolizes caffeine, it can make people feel tired.
The rate at which a person’s body metabolizes caffeine varies depending on genetic and lifestyle factors.
People who drink coffee every day may develop a tolerance to caffeine’s stimulant effects, which means that they need to drink more of it if they want to experience similar results.
While you might have a strong coffee in the morning to wake up, there are some caffeine-fiends out there who sip an espresso after dinner to help them relax and go to sleep.
So how can one stimulant drink – coffee – make some people so alert that they feel anxious and wide-eyed awake for hours and perform no observable effect on others?
The answer has to do with our level of caffeine sensitivity. Our level of caffeine sensitivity is determined by how efficient our body is in processing and metabolising caffeine.
Gabrielle Maston, Accredited Practising Dietitian and Spokesperson for the Dietitians Association of Australia, explains that our genes determine our individual reaction to caffeine and the impact that a specific amount of coffee has on our body.
“Our sensitivity to caffeine varies person-to-person,” says Maston. “So not everyone can consume the same amount of coffee and share the same effect.”
Around 95 per cent of caffeine is metabolised in the liver via an enzyme called CYP1A2, according to the genetics analysis company Gene Planet online. The activity of this enzyme is highly variable and is regulated by the CYP1A2 gene.
“Our sensitivity to caffeine varies person-to-person. So not everyone can consume the same amount of coffee and share the same effect.”
Differences in our DNA will, therefore, produce differences in the rate of our caffeine metabolism – people with more than one C variant in the gene will be more sensitive to coffee and it will yield a stronger effect on them, while those with two copies of a gene present in the A variant will be more tolerant to caffeine.
Maston adds that caffeine sensitivity is also regulated by the receptors in our brain.
Caffeine passes through the blood-brain barrier and influences our brain's adenosine receptors. People who lack the correct adenosine receptors may not experience the same caffeine-related effects of alertness that others do, because the receptors are less responsive and the caffeine molecules cannot bind to receptors.
“That’s why there are people who drink one cup of coffee and are anxious or shaky while others drink six or seven cups of coffee and are fine,” adds Maston.
Research from The Harvard School of Public Health also shows that our genetic differences change the way that various people process, metabolise and become addicted to coffee. The study published in 2014 looked at around 100,000 people from European and African-American cultures and found six genetic variants determining the way coffee habits are formed and variability in how coffee affects our health.
Can I develop a caffeine tolerance?
Maston explains that separate to caffeine sensitivity (which is hardwired into our genes), we may also develop a habitual caffeine tolerance.
“You can build up a tolerance to caffeine, just like you can build up a tolerance to sweetened foods," Maston says. "You acclimatise your tastebuds to the taste of sugar and fat to the point that you don’t taste the sugar and fat in food as much anymore. It’s the same thing with caffeine.
“Take, for example, some people who are Italian or Greek who drink a lot of coffee, socially. Their bodies will most likely become desensitised to the caffeine over time.
“If culturally, you drink coffee at breakfast, lunch and dinner then you will be able to tolerate coffee more than someone who has it sporadically.”
Is caffeine sensitivity something to worry about?
Maston stresses that caffeine sensitivity is not a health condition.
“There’s nothing wrong with one person who is sensitive to caffeine and another person who isn’t – they will just like to drink different levels of coffee.”
However, she says it is wise to be aware of your individual sensitivity levels so you don’t experience the unwanted side effects of drinking too much caffeine – irritability, nervousness, anxiety, sleeplessness and diarrhoea.
Maston also advises that people who are very sensitive to caffeine to switch to tea, which has “about 10 per cent of the amount of caffeine that a regular coffee has”.
“If, culturally, you drink coffee at breakfast, lunch and dinner then you will be able to tolerate coffee more than someone who has it sporadically.”
So how much coffee should I have?
Your recommended level of caffeine consumption is dependent upon the degree of your caffeine sensitivity.
There are three levels of caffeine sensitivity: hypersensitive (very sensitive to caffeine), hyposensitive (minimally or not responsive to caffeine at all) and a ‘normal’ sensitivity to caffeine (which lies somewhere in the middle of the former two categories).
Maston says the majority of people have a normal sensitivity to caffeine and it’s upon this level of sensitivity that recommendations for coffee consumption have been set.
“It’s recommended that most people can have 300-400 milligrams a day or less of caffeine,” says Maston.
“That’s equivalent to around three-to-four cups of instant coffee a day or less. Or, because espressos have higher levels of caffeine content, it’s equal to one-to-two espresso or percolated coffees a day.”
She says most energy drinks combine high doses of caffeine with sugar: “one of those large cans has around three or four shots of coffee in it”.
“We don’t recommend that people drink energy drinks regularly because it can cause issues for your weight control and has strong caffeine content.
“But drinking coffee can be part of a healthy diet and is no problem at all.
“The main message is to have caffeine in moderation. If you are not too sensitive to caffeine, try to stick to the recommended amounts and be sure that you are not adding things like cream and sugary syrups to your coffee because then the sugar content starts to add up.”