If they are 12 hour shifts, so that the people can head home at 5 with everyone else. If they are 8 hour shifts, usually for the later shifts to have an okay life balance: 5-2 for day shift, 2-11 for second shift, and a third shift option that overlaps.
Even though I'm a night person, 5 is a common time to wake up for enough people who presumably want to be productive, and the benefit of getting off work before the school day ends has to be enticing. And on the second shift side of things, they get to have lunch with loved ones before going to work, and 11 is early enough that they could potentially go out for drinks or other fun before bed.
It's also nice that for either shift, the person has time to run errands at a time when most stores are open and activity levels are low.
The machines would wear out faster with more usage, so using them longer each day wouldn't help with that, but doing two shifts per day would increase productivity per machine than one shift per day. Which would also help pay off any loans for the machinery faster.
This is why routine Preventative Maintenance is important. My job strives for World Class Manufacturing, so I hear all the manufacturing buzzwords all day everyday.
Often factories want to get the maximum use out of their equipment since it's very expensive and only lasts some finite number of years. Many factories run 24 hours a day in rotating shifts to minimize the downtime of their machinery.
I used to work EMS 12 hour shifts 6-6. At one point management decided to stagger base start times. Some 6-6, others 7-7 & 8-8. So when someone flips their car and flies out at 5:45 they don’t have to wait for all the available service to finish their shift change, when the 8-8 team has 2 hours left they can get to the patient and back to base without extending their duty day too far.
Many of the workers who went 8-8 hated it. They complained nonstop about how it ruined their whole day since day shift they are headed out the door before the fam eats breakfast and get home after dinner. Same for night shift, rush through dinner or miss dinner and breakfast with the family.
Personally I enjoyed the 7-7/8-8 shifts more but enough people bitched enough they changed all but 1 base back to 6-6.
My old factory job was a 24h operation. Going 5-5 makes more sense than going 8-8 because managers that want to talk to both teams would rather stay until 6 than come in at 7.
8-8 also makes doing anything outside of work nearly impossible on work days. You can generally find doctors, dentists, mechanics, etc. Open after 5, but finding them open before 8 is generally harder.
Not necessarily making them easier, but making it so that one shift worth of management and office/engineering staff can effectively support 2 manufacturing shifts.
The managers could come in at 5, but then they only have a couple hours at best to interface with 2nd shift
ITT: Half of the people doesn't understand what OP asks. The question is not why they start early, it's why specifically 5AM. Why not 5:30? Why not 4? What is the specific significance of 5AM?
My thought is, why not 8am? Why not a variable start time to get the most productive 8 hours of each worker? Why push the start time so much earlier than the "9-5" that is such a regular part of the American vocabulary?
It has to do with balancing home life with work on a rotating shift. Getting out of work at 5pm allows time with the family in the evening. Arriving home at 8pm pretty much guarantees you won't see them because you'll be asleep on night shift until right before your shift. Folks prefer 4am over 5am for this reason.
In my opinion, it's the other way around. The busses run at 4 because thats when they're in demand, and they're in demand because of the time the factory expects the workers to show up.
Where I used to live the bus that took people to the industrial section of town only went there 3 times throughout the day.
It was particularly frustrating for me cause it was the only bus to take you remotely close to the courier's office to pickup a package. But its schedule appeared to be timed with the changes in shifts at the factory.
My uninformed guess would be related to shipping and business hours. If they ship to businesses, then they gotta be up and running before their customers, right? I guess it depends on the industry but maybe the common practice where it is needed can carry over to where it's optional.
I used to be an Operations Manager at a machine shop with 150 employees and a Program Manager at another places with 250 employees. Both had 5am starts. In my experience, the biggest factor was support for the billable staff. You have direct labor employees; these are the people who run machines and fabricate products. Then there are indirect employees that support the direct employees; like purchasing, planning/scheduling, management, customer service, quality, etc. Most manufacturers with a 5am start time are running multiple shifts of direct labor. The indirect employees usually don't start until 7-8am and overlap both shifts to have some support for both day and night shift.
Manufacturers that run one shift(like my current job) usually start later. We start at 7am but allow people to flex their start time for kids, etc.
I did 10 hour shifts 1 hour or 1 hour banked for a few years from 4am. The latest start time to avoid being stuck behind a 4 day long train and get home before traffic hits standstill for hours that worked for everyone that needed to be on site for the same shifts. Probably something similar I'd say because while the shifts were different in later jobs many featured seasonal meetings to discuss adjusting the shift times for the same reasons.
This must be a regional thing. In my area, most places like that run from 7-3:30. I did see a place that ran an area from 5-1:30, but that was specifically during the summer months to avoid the heat.