The holder of an Private Pilot Helicopter License or Daylight Only Restricted Commercial may add the Night Rating to their license and then act as pilot-in-command or co-pilot for any type of helicopter endorsed on one's license by night as well as day.

As required by license held
 

A minimum of 20 hours of flight training in helicopters which shall
 include the following:
      10 hours training at night, including;
             5 hours dual, including 2 hours cross country training
             5 hours solo flying, including 10 take offs and landings 
      10 hours instrument training