You could try the NTSB website. They calculate accident rates for helos each year, and to do that, they gather stats for the number of hours flown each year. However I'm not sure if the hours are broken down into night hours.
Here's another possible source, a private not-for-profit organization called RTCA Inc., with a mission to advance aviation electronics. This includes a lot of NVG work for both military and civil applications. They may have the stats you're looking for. You'll have to browse the site for info. This link came from the Helicopter Directorate of the FAA Certification Office.
http://www.faa.gov/avr/air/asw/rtca/rtca.htm
[ 09 August 2001: Message edited by: Flight Safety ]