You want to be careful how you bring up this issue. Throwing it in their faces that you were promised only 3 months of night-shifts, but have been working them for 6 is a pretty bad idea. (aka don't be aggressive about it when you first bring it up)
I would set up a meeting with the manager and tell him that it was your understanding that working night shifts was a temporary arrangement, and that it is simply not something which you can continue doing due to health issues, life-work balance, or whatever your reasons might be.
If they try to blow you off, or renew this arrangement then you will have to push back more aggressively, and state that you are simply unable to continue working night shifts.
Be prepared that they may not like that, and that you may have to start looking for a new job.