How is overtime calculated with shift diffs?

I know it may be different at different hospitals, but I've been wondering...how is overtime calculated with shift diffs and all? Let's say, for instance, my starting pay is $20.00/hr and I get a $3.00 shift diff for nights and an additional $3.00 for weekends. If I work 24 hours during the week on nights (making $23/hr) and I work 24 hours on the weekend on nights (making $26/hr), how is the overtime calculated? Is it based on the amount I am currently earning after the 40th hour worked for the week? Or would I only get overtime for my base pay (only $10 extra/hr regardless of shift)? Or is it calculated some other way? Does all of this make any sense?? Also, do most hospitals consider Monday to be the first day of the week?

Thanks!