Me too! It’s so frustrating. I worked out what it is though. I have bursts of intelligence when my mind is clear, I.e. I haven’t read a lot or thought a lot for a while and my surroundings have been quiet and calm. The more I think, learn process the more muddle headed I become. Give me a day of meditation and a good nights rest and I’m sharp as a razor for a good few hours of very intense work, conversations etc. But then I fill up and all that new stuff needs to be processed and integrated. I believe other people are the same without realising it. It gives people with a light workload a significant advantage in meetings. Given that I mostly obsessively research things on the internet that makes me pretty fuzzy headed most of the time.