According to Pollan, for most people, there's still enough in your system to disrupt your natural sleep; your brain doesn't shut down enough to get a good rest. IIRC, he was discussing monitoring brain activity during sleep and you can see the difference between people who drank coffee that morning, and people who didn't. Caffeine has just about left your system completely around the time you wake up and start drinking it again, which usually isn't too long before you start getting caffeine withdrawal symptoms.
edit: half life for caffeine is 2-12[1] hours (most sources claim ~5), depending on the person. For a half life of 6 hours (using this because it's close to 5 and divides evenly into 24), you still have >1/8 of what you drank that morning in your system when you go to bed, and 1/16 when you wake up. Over the course of a lifetime, it's not hard to imagine that having a cumulative effect.
According to Pollan, for most people, there's still enough in your system to disrupt your natural sleep; your brain doesn't shut down enough to get a good rest. IIRC, he was discussing monitoring brain activity during sleep and you can see the difference between people who drank coffee that morning, and people who didn't. Caffeine has just about left your system completely around the time you wake up and start drinking it again, which usually isn't too long before you start getting caffeine withdrawal symptoms.
edit: half life for caffeine is 2-12[1] hours (most sources claim ~5), depending on the person. For a half life of 6 hours (using this because it's close to 5 and divides evenly into 24), you still have >1/8 of what you drank that morning in your system when you go to bed, and 1/16 when you wake up. Over the course of a lifetime, it's not hard to imagine that having a cumulative effect.
1: https://www.sleepfoundation.org/nutrition/how-long-does-it-t...