If you are looking for early reports, there was the Bader Report back in the 1970s:-
https://www.flightglobal.com/FlightP...20-%201776.PDF
Here is a GAPAN (now HCAP) paper which may have some useful references:-
http://www.gapan.org/file/2418/aircrew-fatigue.pdf
Access to the Bader Report could be gained through the National Archives:-
Report of the Committee on Flight Time Limitations, chaired by Group Captain Douglas... | The National Archives
I hope these help.