[A2A] You are underestimating the intelligence of professors. It would be a really bad idea for a professor to just look at first few emails of the day and then never read the other emails. If they are not replying to your emails, it means that they are not interested. It does not matter what time you email them. Sorry, these tricks are not going to make them answer to your emails. If you really want a response from the professor, write an email that compels the professor to respond seeing the effort you have put in looking at his work and your interest in working with him. Generic emails usually go to the trash bin.
Coming to your question, my adviser in US used to check his email quite frequently. He would usually reply within an hour or so, if the email required a response. I check my email quite often during the day as well.
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