I just started a new job in June this year and I am finding full time work (5 days per week, 40 hours at work) and motherhood to be so exhausting (my LO is almost 2). I have had three blocked ducts and one case of mastitis since I began my new job! I think exhaustion and stress of trying to do it all are the culprits.
My supervisor is pretty relaxed and there is one other mum on our team who has only worked 3 days per week since she had her baby 6 years ago, but she had been working here for 9 years at that point.. And apparently she had to fight quite a bit for that part time schedule. But that was under different management, so I think maybe it could be different now.
Anyway, I know that I signed up for a full time job but I'm really wondering how long I can keep this pace up. So I'm considering asking in the New Year if it would be possible to drop down to 4 days per week so I could have a rest to look forward to once a week. I will have been with this organization for over 6 months at that point. Also, I honestly think that I could do the same amount of work in 4 instead of 5 days because I have been spending a lot of time on self-study to get up to speed on the content of my role. But I'm not really sure how to address that one (if at all) without sounding like I haven't been doing 5 days worth of work around here...! 
PS. I am posting from Canada but I really like the feedback that MN mums seem to give, that's why I am asking here! But I think we have different laws and rules regarding part time work here than in the UK.