Group by time slots from Unix timestamp

I have date/time saved as Unix time stamp (13 digits). What I am trying to do is to group the rows by time period/range. For example, like below,

    total_order |  time_slot
    24             10 AM - 2 PM
    36             2 PM - 6PM
    30             6 PM- 10PM
    42             10PM - 2AM
    28             2AM - 6AM
    8              6AM - 10AM

I am really drawing a blank on how to approach with this? Any suggestions? Thanks.