What Adenosine Does
Throughout the day, a chemical called adenosine builds up in your brain. Adenosine makes you feel tired and tells your body it is time to rest. The longer you are awake, the more adenosine accumulates. This buildup is a natural signal that sleep is needed.
How Caffeine Blocks Adenosine
Caffeine molecules are shaped similarly to adenosine, so they can fit into the same receptors on brain cells. When caffeine occupies these receptors instead of adenosine, adenosine cannot attach and send its sleepy signal. This tricks your brain into thinking you are not tired. At the same time, caffeine allows other stimulating chemicals like dopamine and adrenaline to work more effectively, making you feel more alert and focused.
Physical Effects You Feel
When caffeine activates your nervous system, your heart rate increases, your blood vessels constrict, and your muscles tense up. You may feel jittery, more focused, and more awake. Your body also releases adrenaline, which increases energy and attention. These physical changes combine to make you feel less sleepy and more ready to work or move around.
Why Tolerance Builds Over Time
If you drink caffeine regularly, your body adapts by creating more adenosine receptors. This means you need more caffeine to block the same number of receptors and get the same wakeful effect. This is called tolerance. Taking breaks from caffeine can help reset your body's sensitivity to it.
Timing and Effectiveness
Caffeine enters your bloodstream quickly and reaches peak levels in your blood 30 to 60 minutes after you consume it. However, you may start to feel its effects in as little as 15 minutes. The effects of caffeine last about 3 to 7 hours, though some people process it faster or slower depending on genetics, medications, and other factors. 
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