Why Does Coffee Sometimes Boost Energy and Other Times Make You Tired?

Lifestyle

Many people know this paradox very well. One day, morning coffee gives you a clear mind, good mood, and energy. Another day, coffee leaves you feeling tired, anxious, or as if your brain is not functioning properly. Some people can even fall asleep after drinking coffee.

How is this possible?

It turns out that the effects of caffeine do not depend only on how much coffee you drink. Your brain chemistry, genetics, stress levels, sleep, hormones, and even the timing of your coffee all play a role.

Coffee Does Not Create Energy — It Masks Fatigue

This is one of the most important points many people do not know. Caffeine does not create new energy in the body. Instead, it blocks adenosine receptors in the brain. Adenosine is a substance that builds up in the brain throughout the day and causes sleepiness and fatigue. The longer you stay awake, the more adenosine accumulates. 1

When caffeine “covers” these adenosine receptors, the brain can no longer properly receive the fatigue signal. The result is a more alert feeling, faster reaction time, and improved focus.

But here comes the paradox. The fatigue itself does not actually disappear. Adenosine continues to build up. Once the effects of caffeine begin to wear off, all the accumulated adenosine can suddenly bind to the receptors at once. This can lead to an abrupt energy crash, drowsiness, or the feeling of being even more exhausted than before drinking coffee.

Why Can Coffee Make Some People Sleepy?

It sounds contradictory, but scientifically it makes perfect sense.

1. You May Be Chronically Sleep-Deprived

If the body is carrying a significant sleep debt, caffeine may temporarily improve alertness and performance, but it cannot replace sleep or fully restore the functions impaired by sleep deprivation. A 2020 systematic review found that caffeine may temporarily improve attention, reaction time, and physical performance after sleep loss, but the effects depend on the dose and the type of task being performed. In other words, coffee may temporarily mask fatigue, but it does not replace sleep or restore the body’s true recovery needs. 2

2. Blood Sugar Fluctuations

Drinking coffee on an empty stomach may cause blood sugar fluctuations in some people. Initially, adrenaline and cortisol levels rise, but this can later be followed by a drop in energy levels. People who are already stressed, sleep-deprived, or eat irregularly tend to be especially sensitive to this.

3. Genetics Affect How You Metabolize Caffeine

This is one of the most fascinating discoveries of recent years.

Caffeine is broken down in the liver by an enzyme called CYP1A2. In some people it works quickly, in others more slowly.

Fast metabolizers:
• feel the effects of caffeine for a shorter period
• often tolerate more coffee
• are more likely to feel energized from coffee

Slow metabolizers:
• keep caffeine in their bloodstream much longer
• experience anxiety, heart palpitations, and sleep problems more often
• may feel more exhausted or overstimulated after drinking coffee

It is estimated that around half of the population are slow caffeine metabolizers. 3

Coffee and Stress Hormones — Why Can Coffee Make You Feel Anxious?

In addition to stimulating the nervous system, caffeine also increases the production of cortisol, the body’s primary stress hormone. This means the heart beats faster, attention sharpens, and the body enters a “readiness mode.” When someone is well-rested and under low stress, this can feel like a pleasant energy boost. But if the body is already stressed, coffee may intensify the situation and create internal restlessness, leaving a person emotionally drained by the evening.

This tends to affect people especially strongly if they:
• sleep too little
• drink coffee on an empty stomach
• are naturally more anxious
• are going through hormonal changes

Why Does Morning Coffee Sometimes Stop Working?

One reason may be tolerance. Regular caffeine consumption changes the sensitivity of the brain’s adenosine system. The brain may start producing more adenosine receptors to balance out caffeine’s effects.

As a result:
• the same amount of coffee produces a weaker effect
• fatigue feels stronger without coffee
• a person gradually needs more caffeine

This does not necessarily mean addiction, but studies clearly show that chronic caffeine consumption leads to neuroadaptations — the brain adapts to constant adenosine receptor blocking. This helps explain why the same amount of coffee becomes less effective over time. 4

Timing Matters

Caffeine’s half-life averages around 3–7 hours, but for some people it can be significantly longer. This means that even coffee consumed in the afternoon may still affect the body late into the evening. For example, caffeine from a coffee consumed at 3 PM may still be active at night and affect sleep quality, even if the person feels they fell asleep normally.

A 2023 systematic review found that caffeine may reduce total sleep duration and deep sleep while increasing nighttime awakenings. 5 This can create a vicious cycle: poor sleep increases fatigue, fatigue increases caffeine cravings, and higher caffeine intake further worsens sleep.

So, Is Coffee Bad for You?

Not necessarily. Moderate coffee consumption can be a completely normal part of a healthy lifestyle for many people, and studies have linked it to improved attention, physical performance, and even a lower risk of several neurodegenerative diseases. The problem is usually not coffee itself, but rather how much is consumed, when it is consumed, and how an individual’s body reacts to caffeine.

The European Food Safety Authority (EFSA) considers up to 400 mg of caffeine per day safe for most healthy adults. This corresponds to roughly 3–5 cups of coffee, depending on the strength and serving size. During pregnancy, caffeine intake is recommended to stay below 200 mg per day.

Still, a “safe amount” does not mean it suits everyone equally well. Some people may experience anxiety, heart palpitations, or sleep disturbances after just one cup, while others can drink several coffees a day without noticeable effects. Genetics, stress levels, sleep, hormonal changes, and overall health all play major roles.

The most important thing is to pay attention to how your own body responds to coffee. If coffee improves your mood and focus without negatively affecting sleep or well-being, there may be no reason to avoid it. But if caffeine causes anxiety, energy crashes, or sleep problems alongside fatigue, the issue may not be the coffee itself — it may be that your body actually needs more recovery.

Coffee Enjoyment With Added Benefits

For many people, coffee is more than just caffeine — it is part of a morning ritual and a small moment for themselves. Why not make that habit a little more nourishing and functional? Be More coffee collagens combine high-quality coffee and collagen in one drink. In addition to the pleasure of coffee, you can also support the normal condition of your skin, joints, and connective tissues.

Because sometimes the question is not just whether coffee energizes you — but what your body truly needs alongside it.

Used sources:

¹ J Sleep Res. 2022 May. Adenosine, caffeine, and sleep–wake regulation: state of the science and perspectives

² C. Irwin, S. Khalesi jt. 2020 Jan. Effects of acute caffeine consumption following sleep loss on cognitive, physical, occupational and driving performance: A systematic review and meta-analysis.

³ S. Mahdavi, P. Palatini jt. 2023 Jan. CYP1A2 Genetic Variation, Coffee Intake, and Kidney Dysfunction

⁴ Yu Zhao Lee, Ker Ni Chua jt. 2026 Jan. Neurobiology of chronic caffeine use and withdrawal: Mechanisms, effects and implications

⁵  C. Gardiner, J.Weakley jt. 2023 June. The effect of caffeine on subsequent sleep: A systematic review and meta-analysis