Depends but possibly freight and logistics cycles.

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.

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.

If they’re running 24hrs/day with 3 shifts, gives graveyard shift enough time to get home before their families wake up.

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.

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?

Because the buses start running at 4am.

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.

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.

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.

It's about money. Early start, early finish usually. But if overtime is needed, there is time in the day for it.

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.

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.

I would guess that you have to run the air conditioner less if you get to work earlier.