|"Is it okay to turn your router off at night or when not using the internet access, say for a week-end while gone"|
Yes. Why would you want to though?
"Is there any loss of assigned IP address..."
Yes. You will most likely get a different IP address from your ISP when you turn the router back on after a week.
"Also, does anyone know of any health hazards having router on..."
Zero. You are literally awash in radiation all the time. Adding a wireless router to the mix, even assuming that radio devices pose a health hazard (which I would argue is false), is insignificant.