We know that when we sleep too little, do hard physical or mental work, or stay up longer than usual, we feel sleepy and we’re more likely to doze off. This suggests that some sort of sleep-inducing signal accumulates in the brain, and the longer we’re awake, the harder we work, the more it builds up. We now have strong evidence that a chemical called adenosine is that signal.6 Adenosine builds up in the brain while we’re awake, and it builds up even faster when we exert ourselves.7 As it accumulates, it begins to inhibit the arousal system and activate the VLPO. Eventually, when adenosine
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