It is the adenosine system that so many of us trick with our daily dose of caffeine. When adenosine levels are low, acetylcholine is released in the basal forebrain. As we saw in Chapter 2, acetylcholine guides our attention towards external stimuli, making us feel awake and alert. Adenosine prevents the release of acetylcholine, so we feel sleepy. But caffeine blocks the receptors that are normally triggered by adenosine, making it seem to our brains that the concentration is lower than it actually is. This, in turn, means more acetylcholine is released, activating neurons in the cortex and
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