system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why can you feel tired at 8pm (so tired you could literally fall asleep at any moment) but when you actually go to bed you can't fall asleep?
gpt
We're going to have to make some assumptions about your nightly routine here, but roughly - 

Your brain can switch off while watching tv or movies, as they require no real mental effort usually. Once you decide to go to bed though, you have to get up, walk around, turn lights on and off as you brush your teeth, go to the toilet etc, and generally re-stimulate yourself. Once you *are* in bed, you have no mental white noise to keep your thoughts and brain activity suppressed, and so rebound into normal wakefulness. You then have to wait for your brain to realise that "hey, it's dark in here. Must be night time after all" and start its shutdown process.