I asked to be trained in a different role once, the only time I could do it (taking deliveries) was to do the 3am-11am shift. I was doing a lot of overtime on top of my regular hours, so I did a 1-9pm shift, then the 3-11 training, then another 1-9. I made a few mistakes on that final shift on account of being unbelievably tired and received a verbal warning for being a bit rude to a customer, which although I take full responsibility for I would have appreciated some understanding from my manager. I decided to cancel any further training and refuse all overtime going forward as I genuinely thought I was going to die at one point, this was 7 years ago and I've still not worked longer than a few minutes past the end of my contracted time.