There is not a perfect answer for this question. Most 8-hour operations have the day shift start between 6:00 am and 7:00 am. The afternoon shift would start 8 hours later; the night shift, 8 hours earlier. for 12-hour shifts, the start times tend to start about 30 minutes earlier. So, if you are on an 8-hour schedule that has a day shift that starts at 6:30 am, expect the workforce to want a 6:00 am start time for 12-hour shifts.
Our research has shown that employees starting at 7:00 am get about 20 minutes more sleep per night than those starting at 6:00 am. Before you run out and change your schedule, consider the following: (1) shiftworkers are typically locked into whatever start time you currently have. They will resist change. (2) The later the day shift starts, the later the night shift gets off. This is the trade-off. Ideally, a night shift would end early enough to allow the night shift to get home before the sun comes out. This means getting off earlier rather than later.