I believe it is really about which part of the world is awake at that moment and which parts are sleeping. Considering ethereum (and by in large crypto as well) is a global thing I am afraid it is never really asleep, crypto is always awake, if one nation goes to sleep, that means other nation is waking up so it is a 7/24 market that never gets low.
Definitely there could be some "smaller" periods but the reality is that the difference is not big enough to chase those minutes neither, it is definitely something that is too small to decide if it is worthy or not. I personally would trade anytime I want if you ask me because I would rather act quickly before I could try to find a better time. Right now I have a lot of small coins that worth nearly nothing compared to the network fee, so instead of trying to chase that, I am trying to just figure out how ETH fee could ever go down.