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Coffee is the most widely consumed beverage globally, so it’s no wonder that many people can’t imagine starting their day without it. The sound of a coffee machine is often heard in households before anyone has even fully opened their eyes. The main active ingredient in coffee is caffeine, widely regarded as the most commonly used legal drug. Its effects are familiar to anyone who enjoys a good cup of coffee or needs a boost during a busy day. Caffeine is a stimulant, making it popular not just in daily life but also in the world of sports.
In recent years, morning coffee has been heavily criticised. A theory, particularly widespread on social media, claims that drinking coffee immediately after waking up is unwise. It suggests that this practice can lead to afternoon fatigue and disrupt the circadian rhythm due to its impact on cortisol levels. This theory is based on the idea that coffee can interfere with the body’s natural circadian rhythm. But is there any evidence to back this up? And how does caffeine really affect our biological clock?
Caffeine and Its Effects
Caffeine is a substance naturally found in coffee beans. It’s an alkaloid that originally served as a defence mechanism for the coffee plant against pests. Its effect on insects can be lethal. For humans, it only becomes lethal at doses above 10 grams of pure caffeine. Caffeine has the ability to bind to adenosine receptors. To understand its mechanism of action, it’s important to define adenosine and its role in the 24-hour cycle.
Adenosine builds up in the brain as we use energy throughout the day. When the brain is working at full capacity, a large amount of ATP is consumed, leading to the production of adenosine. Throughout the day, adenosine accumulates in the brain, signaling that it’s time to sleep when it reaches high levels. During sleep, adenosine levels decrease, as the brain isn’t as active as during the day. However, during REM sleep, when the brain generates dreams, adenosine levels remain slightly elevated. [1]
When adenosine binds to its receptors, it causes fatigue. However, when caffeine occupies these receptors, adenosine cannot bind, thus delaying the feeling of tiredness. It’s important to note that caffeine does not provide energy; it merely postpones fatigue. [1]
A viral theory suggests that drinking coffee immediately after waking up blocks adenosine receptors, causing adenosine to accumulate throughout the day. Once the caffeine leaves the receptors, the accumulated adenosine suddenly binds to them, which is believed to cause afternoon fatigue. This theory assumes that adenosine levels decrease upon waking up and then build up gradually during the day.
In reality, adenosine levels increase exponentially within the first few minutes of waking up. By the time you reach the coffee machine, your brain is already flooded with adenosine, well before you take your first sip. Therefore, delaying your morning coffee does not prevent afternoon fatigue. [2]
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How Does Caffeine Affect Cortisol?
Caffeine also influences cortisol levels. While cortisol is known as the stress hormone, it plays a crucial role in the body’s 24-hour cycle. Cortisol levels are lowest just before sleep and begin to rise around 3:00 AM, peaking roughly an hour after waking up. [3]
This hormone signals the body to wake up. Caffeine increases cortisol levels by approximately 30% within 60 to 120 minutes of consumption, partly due to the accompanying rise in adrenaline. Caffeine not only delays fatigue but also enhances alertness. [4]
However, this increase in cortisol does not occur in people who regularly drink coffee. Even when it does, there is no evidence that caffeine disrupts the natural circadian release of cortisol. [5]
Is It Safe to Drink Coffee on an Empty Stomach?
One common concern is whether it’s safe to drink coffee on an empty stomach. Some believe it increases the risk of stomach ulcers by boosting the production of stomach acid, which, without food, might irritate the stomach lining.
Research suggests that drinking coffee on an empty stomach is generally safe, provided you don’t have pre-existing conditions like stomach ulcers, irritable bowel syndrome, or acid reflux. [6]
So, What’s the Conclusion?
Drinking coffee right after waking up does not cause adenosine to accumulate throughout the day. By the time your coffee is ready, your brain’s adenosine levels are already high, and temporarily blocking its receptors with caffeine won’t lead to a sudden energy crash in the afternoon.
Caffeine also has the ability to “push” adenosine out of its receptors and replace it. After waking up, caffeine doesn’t block receptors and cause adenosine to build up; instead, it occupies receptors that already have adenosine. Caffeine delays fatigue, and once its effects wear off, adenosine gradually takes over. Delaying your morning coffee may push back afternoon fatigue, but it might also lead to a need for another cup later in the day, which could negatively impact your sleep quality.
The effect of caffeine on cortisol levels is similar to the impact of intense exercise. From the wealth of data and studies available, we know that intense exercise does not disrupt the human circadian rhythm. The same goes for caffeine. Its effects don’t kick in until about an hour after drinking coffee, long after you actually drink it. For regular coffee drinkers, the cortisol response is almost completely suppressed.
In a time when many people suffer from poor sleep quality, having a cup of coffee immediately after waking up can be a valuable tool to help you get up. Ultimately, it comes down to personal preference and individual response. Some people wake up feeling refreshed, while others struggle to open their eyes. Genetics and habits play a role in this but don’t forget that improving sleep quality can also help, with melatonin being one possible aid.
[1] Reichert, C. F., Deboer, T., & Landolt, H. P. (2022). Adenosine, caffeine, and sleep-wake regulation: state of the science and perspectives – https://doi.org/10.1111/jsr.13597
[2] Murillo-Rodriguez, E et al. “The diurnal rhythm of adenosine levels in the basal forebrain of young and old rats.” – https://doi.org/10.1016/j.neuroscience.2003.09.015
[3] Krieger, D T et al. “Characterization of the normal temporal pattern of plasma corticosteroid levels.” – https://doi.org/10.1210/jcem-32-2-266
[4] Lovallo, W R et al. “Stress-like adrenocorticotropin responses to caffeine in young healthy men.” – https://doi.org/10.1016/S0091-3057(96)00105-0
[5] Lovallo, William R et al. “Caffeine stimulation of cortisol secretion across the waking hours in relation to caffeine intake levels.” – https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2257922/
[6] Nieber, Karen. “The Impact of Coffee on Health.” – https://www.thieme-connect.com/products/ejournals/abstract/10.1055/s-0043-115007
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