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For optimal brain gain, regular tea breaks, as favored in the UK, are more effective than a 20-ounce French roast... Throughout the day, your noodle fills up with adenosine, a chemical thought to cause mental fatigue. Caffeine blocks the brain's adenosine receptors, countering the chemical's dulling effects. To maximize alertness and minimize jitters, keep those receptors covered with frequent small doses — like a mug of low-caf tea or half a cup of joe — rather than a onetime blast.