Framer, thanks for the recognition but Atcos like millions of other people, including pilots,
work during the day and during the night. I never heard of problems due to circadian cycles
in my time in the air and in Atc. In Atc one of the skills is in knowing what the aircraft are going to say before they say it. It works, most of the time. Also it is unlikely that even a single controller on duty in the Tower will be alone. Alongside him/her will be a clued-up assistant.