Coffee or tea? The effect of caffeine on sleep and circadian rhythm

17.12.2025

Inspiracja

Coffee or tea? The effect of caffeine on sleep and circadian rhythm

Coffee and tea are an inseparable part of the day for many people — they provide energy and are often associated with a moment of rest. However, their effects go far beyond a short-term boost. Caffeine — a compound present in both beverages — can significantly influence sleep, the quality of recovery, and the body’s natural circadian rhythm. How strongly does caffeine affect our biological clock, when should you drink your last cup of the day, and why might tea sometimes be the better choice?

What will you learn in this article?

Caffeine — a natural compound with strong effects
How it works in the body
Caffeine and its effects on the brain
Its impact on recovery
Genetics and caffeine sensitivity
Coffee vs. tea
When to drink your last cup of coffee—and why it matters
How to include coffee and tea wisely throughout the day
Key takeaways

Caffeine — a natural compound with strong effects

Coffee and tea can be a healthy part of the day when consumed in harmony with the body’s biological clock. Caffeine is a great servant but a poor master — it energises when needed, but if these beverages are consumed at the wrong time, it can disrupt recovery, sleep, and hormonal balance.

Caffeine is a natural alkaloid that acts as a stimulant of the central nervous system. It is found mainly in coffee beans and tea leaves. It works by blocking the effects of adenosine, a molecule in the brain responsible for inducing feelings of tiredness. Because of this, caffeine increases alertness, improves concentration, and temporarily enhances physical and mental performance.

The effects of caffeine typically appear within 15–30 minutes after consumption and usually last for several hours (typically 4–6). However, sensitivity varies depending on genetics, age, body weight, and habitual intake.

In higher amounts, caffeine may cause insomnia, nervousness, a faster heart rate, or digestive discomfort. Therefore, it is recommended to monitor daily intake (up to approx. 400 mg for an adult, equivalent to 3–4 cups of coffee depending on the brewing method).

How it works in the body

Caffeine stimulates the hypothalamus and adrenal glands, which increases the release of the stress hormone adrenaline, activating the body into the sympathetic “fight or flight” mode. Caffeine therefore boosts energy but can also worsen anxiety and sleep if consumed in excess. This hormonal cascade also leads to an increased heart rate, higher blood pressure, greater blood flow to the muscles, and heightened tension and stress sensitivity. Thanks to these effects, caffeine can paradoxically improve performance during exercise or endurance sports.

With regular coffee consumption — and to a lesser extent tea — the body gradually adapts to caffeine, and the brain increases the number of adenosine receptors. As a result, the effect of caffeine decreases over time, requiring higher amounts to achieve the same stimulation. This can affect mood and concentration because the body begins to demand increasingly higher doses.

Problems arise when the effect of caffeine persists into a time when the body should naturally slow down or prepare for sleep. Excessive or late caffeine intake can shorten the total duration of deep sleep, disrupt melatonin production, or shift the natural circadian rhythm by several hours.

Caffeine can also significantly affect the absorption and metabolism of certain micronutrients, especially minerals. Coffee and tea reduce the absorption of iron, particularly from plant sources, which is why it is better to drink them at least one hour after meals. These beverages may also increase the urinary excretion of calcium and magnesium, so with higher coffee consumption it is advisable to replenish these minerals. For most other minerals, the effect is minimal. Caffeine also has a mild diuretic effect, which may increase the loss of B vitamins and vitamin C.

Caffeine and its effects on the brain

Caffeine is one of the most widely used psychoactive substances in the world. Studies show that moderate, regular caffeine intake may have protective effects on the brain for several reasons. These benefits appear only with moderate consumption — about 1–3 cups of coffee per day.

it may reduce the risk of neurodegenerative diseases:

Caffeine modulates the dopamine system, increasing the body’s sensitivity to this neurotransmitter. This contributes to protection against Parkinson’s disease, where dopamine-producing neurons progressively die.

it improves cognitive functions, not only in older adults:

Mild stimulation of the central nervous system through caffeine improves attention, concentration, and working memory.

it increases neuronal resistance to oxidative stress:

Caffeine has strong antioxidant properties, helping reduce free radicals that damage cell membranes, mitochondria, and neuronal DNA. It also activates protective enzymes (e.g., glutathione peroxidase and superoxide dismutase), which help neutralise oxidative stress. To benefit from these protective effects, the quality of the coffee is essential.

Its impact on recovery

Caffeine has a dual effect on recovery, depending on the dose, timing, and individual sensitivity.

After physical exertion, caffeine can accelerate glycogen resynthesis, thereby supporting the restoration of muscle energy. It also reduces the perception of fatigue and improves concentration after exercise.

On the other hand, caffeine increases levels of stress hormones, adrenaline and cortisol, which mobilises energy, but when consumed excessively or too late in the day, it can impair the body’s ability to shift into the rest-and-recovery mode needed to restore muscles and the nervous system.

Increased coffee consumption also increases mineral loss (especially magnesium), which is essential for recovery. For athletes or individuals exposed to high stress, long-term excessive coffee intake may lead to mineral imbalances in the body.

In summary: moderate consumption of caffeinated beverages (e.g., 1–2 coffees per day, ideally in the morning or around exercise) may support recovery. Higher or late caffeine intake, however, has the opposite effect — mainly due to disrupted sleep and hormonal imbalance.

Genetics and caffeine sensitivity

Each of us responds to caffeine differently. While a morning cup may help one person concentrate, another may experience heart palpitations, anxiety, or insomnia. These differences are not random — genes play a major role in determining how quickly the body metabolises caffeine and how strongly it affects the nervous system.

The gene CYP1A2 encodes a liver enzyme responsible for caffeine metabolism. There are two main variants of this gene:

Fast metaboliser (CYP1A2*1A) breaks down caffeine more quickly, its effects are milder and shorter-lasting. These individuals generally tolerate coffee well and usually do not have sleep issues even when drinking it in the afternoon.
Slow metaboliser (CYP1A2*1F) breaks down caffeine more slowly, which may lead to nervousness, heart palpitations, or sleep disturbances. These individuals have a higher risk of negative caffeine effects, such as increased blood pressure.
Another important gene that influences caffeine’s effects in the body is COMT (catechol-O-methyltransferase). This enzyme breaks down dopamine, adrenaline, and noradrenaline — neurotransmitters associated with focus, motivation, and stress, and whose levels are increased by caffeine.

In the COMT Val/Val variant, the enzyme works more quickly, so these neurotransmitters are broken down sooner. These individuals tolerate caffeine better and typically feel more stimulated.
In the COMT Met/Met variant, the breakdown of neurotransmitters is slowed, leading to higher levels in the brain — caffeine often triggers anxiety, tension, or insomnia in these individuals.

Coffee vs. tea

Although both beverages contain caffeine, their effects on the body differ because tea also contains additional compounds that can moderate caffeine’s impact.

The effects of coffee usually appear quickly — caffeine is absorbed within 15–30 minutes and its blood levels rise rapidly. This results in a strong stimulation but also a potential “energy crash” after a few hours.
Tea, especially green or white tea, contains not only caffeine but also L-theanine, an amino acid that reduces stress and supports focus. As a result, the caffeine in tea has a smoother, longer-lasting effect without sharp energy spikes.

If you need sustained mental clarity throughout the day and want to avoid disrupting your sleep, tea is often the gentler option — especially later in the afternoon or in the evening.

When to drink your last cup of coffee — and why it matters

The ideal time for your last coffee should be no later than 6–8 hours before bedtime. In sensitive individuals, even a small dose of 50–100 mg of caffeine (the amount in one espresso or one cup of filter coffee) can affect sleep quality. If you go to sleep around 22:30, your latest coffee should be around 14:30.

How to include coffee and tea wisely throughout the day

Your first cup of coffee should definitely not be consumed on an empty stomach as the first drink of the day. Cortisol levels are naturally higher in the morning to help us wake up. Drinking caffeine immediately after waking further increases this stress hormone. Coffee also stimulates the production of digestive acids, and drinking it on an empty stomach can contribute to stomach irritation or even ulcers. Ideally, have your first coffee after breakfast or lunch to support concentration and digestion.

In the late afternoon, tea is the better option as it contains less caffeine and naturally provides calming L-theanine.

In the evening, include only caffeine-free alternatives — rooibos, chamomile, lemon balm, or adaptogenic blends with reishi or ashwagandha.

Key takeaways

Caffeine is an effective stimulant that increases alertness and concentration, but with excessive or late consumption it can disrupt sleep and the circadian rhythm.
Moderate caffeine intake (1–3 cups per day) may support cognitive function and provide neuroprotective effects thanks to its antioxidant properties.
Caffeine sensitivity is highly individual — it depends on genes (CYP1A2, COMT), which determine the speed of its metabolism and the nervous system’s response.
Tea has a gentler effect than coffee because it contains L-theanine, which reduces stress and balances the effects of caffeine.
Timing is crucial — drink your last coffee no later than 6–8 hours before bedtime.