Shift start times

I currently work in a hospital where shift start times vary by unit. They are making us all go to one uniform start time. Right now our unit starts at 645 am/645pm with report to be on the floor by 715. It's possible they could push that start time back to 6am/pm which probably means report begins at 545. Does anyone know of any research on shift start times? There are many reasons a start time of 545 would cause a problem in my life, but I would like more information on this.
Shift start times are the facilities policy. In the ED We have shifts that start at 7a, 9a, 11a, 3p, 5p, 7p and 11p

staggered start times make sense in a lot of ways, the only major draw back I see is in floating nurses. therefore I would be prepared for an uptick in that happening...