Hi Sandeep,
You mentioned you would work in 3 shifts -
Night shift for 12 hours (7 to 7) whist the other two for 6 hours (splitting the remaining 12 hours in 2)
Usually in IT industry the dress code is formals for weekdays and business casuals for weekends. Business casual meaning - No shorts / capri, no sleeveless / bandis, no tee-shirts with inappropriate message on it.
You mentioned that there are no females in the night shift. But we need to realize that females might be in the other two shifts. Since you are to work 24*7, there would be some over lap of time allowing the employees of any two shifts have a brief intro or meet while walking in/out.
Appropriate dress code would keep the office formal. Also I believe the night shift would not have an HR. If that is the case, there has to be proper tracking system to track the dressing of the night shifters.
Again I always say, we can only give you suggestions. When it comes to implementation, you need to weigh pros and cons of all the things as per your industry and staff members and act accordingly.