How Caffeine Works

This content was created by the National Sleep Foundation


Learn what caffeine actually does to your body.

While you probably know that caffeine can keep you awake, you may not understand how the substance actually works. Myths abound when it comes to caffeine, and its effects vary from person to person. By understanding how caffeine works, you can use it more efficiently.

How It Stimulates You

While you’re awake, the neurons in your brain fire away and produce a compound called adenosine as a byproduct. Adenosine is constantly monitored by your nervous system through receptors. Typically, when adenosine levels drop and hit a certain low level in your spinal cord and brain, your body will signal to you to start relaxing to prepare for sleep. There are several different adenosine receptors throughout the body, and caffeine most affects the A1 receptor. When a substantial amount of caffeine is ingested—such as the typical 100 to 200 milligrams from a strong, eight-ounce cup of coffee—it works its way toward your body’s adenosine receptors and mimics the effects. In other words, caffeine tricks your body into thinking that it's not yet time for sleep by acting like adenosine.

How Fast You Process It

Though caffeine can give you more energy, it still doesn’t make up for lost sleep. The effects of caffeine—including how long it lasts and how much it boosts your energy—vary from person to person, depending on their tolerance, genetics, and other factors determined by their unique physiology. But generally, caffeine lasts about five to six hours in the body before wearing off. For women taking birth control pills and for women between ovulation and the beginning of menstruation, it takes about twice as long to process the caffeine. For regular smokers, their bodies take half as long to process caffeine.

How Withdrawal Affects You

Whether people opt for an afternoon soda or morning espresso, they’ll typically develop a strong tolerance to a daily dose of caffeine somewhere between a week and 12 days. However, when they stop getting a daily fix, the tolerance goes away. The effects of caffeine withdrawal come on very quickly, between 12 to 24 hours after your last use. That’s why people often crave a cup of coffee in the morning—to ward off withdrawal. The most common symptom is a headache, but symptoms can also include depression, fatigue, lethargy, irritability, nausea, and vomiting. Depending on your body, those will last for about 10 days, until withdrawal is over.