I just found the following paper:
Katia Levecque, Frederik Anseel, Alain De Beuckelaer, Johan Van der Heyden, LydiaGisle, "Work organization and mental health problems in PhD students", Research Policy, 46, 4, 868-879, 2017 (Link
HERE).
which (I think) it is considered as one of the most serious quantitative research studies on the effect of the stress on students associated to carrying out a PhD thesis.
Katia Levecque was very recently interviewed (link
HERE) and it seems they have plans to expand the aims and population of the research published in the
Research Policy Journal. Good news then!
There is a wide range of symptoms called as "mental health problems", including just "depression" and "anxiety", and if that is the case, I had them (who did not?).
In the end, I believe it was worth it for me, but I can only talk for myself...