When did air traffic controllers start one-person night shifts?

I'm guessing that with a strong Air Traffic Controllers union, work rules allowing one-person night shifts would never have been allowed. So my hypothesis is that the problems we've been seeing with sleeping controllers can be blamed, at least in part, on Reagan's busting of the ATC union. But I don't know enough about the history of the ATC union or how work rules are created today to really make this accusation. Can anyone with better historical knowledge of these work rules and the ATC union prove or disprove the Reagan connection to today's problem?