Coffee has been enjoyed for centuries not only as a beverage but also as a tool to sharpen the mind and fight fatigue. The secret ingredient is caffeine, one of the most widely used stimulants in the world.

When you drink coffee, caffeine is quickly absorbed into your bloodstream and reaches your brain within 15–45 minutes. There, it blocks the effects of a neurotransmitter called adenosine. Normally, adenosine builds up in the brain throughout the day and makes you feel tired. By blocking adenosine, caffeine tricks your brain into feeling less sleepy.

This blockage also indirectly increases the activity of other brain chemicals such as dopamine and norepinephrine, which boost your mood, alertness, and focus. That’s why a single cup of coffee can make you feel more awake, motivated, and ready to take on tasks.

The stimulating effect of caffeine usually lasts about 4 to 6 hours, depending on the person. Some people metabolize caffeine faster than others, which is why your friend may be able to drink coffee at night and still sleep, while you might stay awake for hours.