People react to caffeine differently. While many people feel more alert after drinking caffeinated coffee, some might feel tired for several reasons.

Is it really the coffee?

As a stimulant, caffeine can boost energy levels and make you feel sharper. In the United States, the biggest dietary source of caffeine is coffee. About 62 percent of Americans drink coffee every day, according to the National Coffee Association.

Not everyone reacts the same way to caffeine. Some people feel tired after only one cup. Others can drink several cups a day and feel no ill effects.

But it’s not actually coffee that makes you tired. It’s the way it affects your body that can lead to sleepiness. Keep reading to learn more.

Adenosine is a chemical in the central nervous system. It regulates your sleep-wake cycle. When you’re awake during the day, your adenosine levels increase, eventually making you drowsy by suppressing the activity of cells in the basal forebrain. After you fall asleep, adenosine levels drop.

Caffeine in coffee blocks the brain’s adenosine receptors from receiving adenosine, but it doesn’t stop the actual production of adenosine or the ability to form additional adenosine receptors. This means that when the effects of caffeine wear off, there’s a buildup of adenosine wanting to bind to its receptors. This can lead to tiredness.

Caffeine has been considered a diuretic for years. A diuretic is a substance that makes you pass urine more often. This lends itself to the theory that drinking a lot of coffee increases your risk of dehydration.

But many scientists argue that caffeine-containing beverages don’t really impact urinary output in the long term any differently than other beverages.

If you do find that drinking coffee makes you urinate more frequently than normal, you may get stuck in a cycle of dehydration that makes you feel more tired.

First of all, your body loses water when you go to the bathroom. The water loss can reduce the fluid in your blood, which can affect how your cardiovascular system responds to maintain blood pressure and blood flow. Dehydration can lead to a rapid heart rate and low blood pressure. This can lead to feelings of fatigue and sluggishness.

When dehydrated, cells in the body lose fluid volume. When this affects their normal function, it can also lead to feelings of sluggishness. It’s natural to reach for another cup of coffee to counteract this sluggishness, but this can start the cycle all over again.

Caffeine is also causes vasoconstriction. This means it causes certain blood vessels to narrow. This could alter blood flow through different parts of the body.

If you’re drinking a lot of coffee, you may not be drinking as much water as you should to rehydrate yourself. The Health and Medicine Division of the National Academies recommends being guided by your thirst, but does provide a total daily water intake to aim for:

  • 15 cups (3.7 liters) for the average adult male
  • 11 cups (2.7 liters) for the average adult female

This guideline includes water in drinks other than pure water and water from the food you consume. Unless you’re experiencing symptoms of dehydration, such as dark-colored urine and headache, you’re probably drinking enough water.

If you like to add sugar to your coffee, you may have regular sugar “crashes” after drinking it. This added sugar may come in the form of whipped cream or shots of syrup. These are often standard in specialty coffee drinks.

The body processes sugar much faster than caffeine. After sugar is used up by your body, you may experience an energy slump. How quickly this happens depends on the person. It could happen within 90 minutes after ingesting sugar.

If you don’t want to give up your coffee habit, try to stick to the daily intake recommendations.

Up to 400 milligrams (mg) of caffeine per day is considered moderate. This is about two to four 8-ounce cups of brewed coffee per day, depending on the coffee mixture.

To further minimize fatigue, avoid coffee-based drinks with sugary syrups and creams. You should also limit your use of added sweeteners. Alternating one cup of coffee with one cup of water may also help.

If you regularly experience an afternoon slump, try switching to decaf coffee or tea after lunch.

Remember, coffee isn’t the only thing that contains caffeine. Soft drinks, energy boosters, and even some pain relievers contain caffeine. The overall effect of caffeine on your body depends on the total amount in your body from all sources and how frequently you take caffeine in.

Coffee itself won’t instantly make you feel tired, but the caffeine it contains may actually lead to fatigue after regularly drinking it over time. If you stick to 400 mg of caffeine per day or less and go easy on the added sugar, you should reap the benefits of caffeine and avoid its drawbacks.