It sounds like your body doesn't like the 12 hour shifts. Are you off work each evening now? Or do your 8 hours rotate?
The issue is nights are only 12h so asking for nights guarantees 12h shifts, while days are mix of 8h and 12h, and since I'm low on the seniority list I get mostly 8h when I'm on days, so I get less time off. I don't mind the 12h shifts, I think it's the nights themselves that mess with my body. Even though I actually like working nights I think my body does not. But I'm not actually sure if that's the issue yet, just trying this as an experiment. The 12h day shifts are harder due to having to be up earlier, and it's always busy after 5pm, but they also mean more time off, so I still prefer them.
Right now I'm on mostly 8h with the odd 12h, but it means I'm always working instead of getting nice stretches of time off.