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How does caffeine in tea affect the central nervous system?

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Neatly planted rows of green tea bushes stretch across rolling fields with Mount Fuji, capped with snow, rising in the background in Japan. — tea Rows of tea (*Camellia sinensis*) growing in Japan, with Mount Fuji in the background.

Caffeine acts as a stimulant on the central nervous system by inhibiting the effects of adenosine, a chemical byproduct of cell metabolism that regulates sleep.1

Because caffeine is structurally similar to adenosine, it can bind to adenosine receptors in the brain without triggering the same sleep-signaling response. By blocking these receptors, caffeine prevents the body from recognizing the biological signals that indicate a need for sleep, thereby increasing alertness and decreasing fatigue.21

While tea typically contains less caffeine than coffee—averaging about 40 mg per cup compared to 100 mg—it still provides a stimulating effect. This milder stimulation is often preferred by those seeking a gentler energy boost. However, excessive consumption of caffeine from any source can lead to side effects such as jitteriness, anxiety, irritability, headaches, and insomnia.13