First shift can start anywhere from 5 am to 9 am - Depends on the company

Generally, factory work had the shifts, so it went something like this:

First – 7 A.M. to 3 P.M.

Second – 3 P.M. to 11 P.M.

Third (graveyard) – 11 P.M. to 7 A.M. 

These were all local time, but now with people working for the same company in different time zones, first shift can start anywhere from 5 A.M. to 9 A.M. – then just add 8 hours for second, and 8 hours for third. 

For medical transcription, it is whenever they need someone to cover so there are enough MTs working for TAT.  A lot of on-line applications now ask what hours you are available or want to work.  If you limit yourself to a certain timeframe, they may pass you over for someone more flexible or who fits their needs exactly. 

First Shift (Day Shift) could coincide with the available of office and executive staff.  So, if the big wigs at the company that is located on the East Coast arrive for first shift at 7 A.M., but you live on the West Coast, you could be starting your shift at 4 A.M. to coincide with their time schedule. 

A lot of companies are either listing the exact hours they need covered, or are now calling them A.M., Day, or Morning Shift / P.M. or Afternoon Shift / Overnight Shift.