Hey, GeorgeK -
If it's a light-intensifier type scope (most are), you could only use it during the day with a very dark filter. Some of the older scopes had pinhole lens covers for use during daylight, but as I recall the image resolution wasn't great and the field of view was much narrower than you could get at night.
A thermal or IR sight (two different things) should work equally well day or night, but those are not common on rifles; thermal sights are large and heavy because they require cooling systems to work, and the IR devices I've seen have had a pretty short range. You could mount a thermal sight on a tripod-mounted machine gun if it's going to be in one place for a while, but I wouldn't want to hump one around with my rifle.
Thermal sights are usually hooked up to a vehicle or other power source, but some can run on batteries. The others all use batteries.
If you are in the U.S. Army and you drop your weapon, you'll need to drop with it and do push-ups. If you drop it with optics mounted, you'll want to re-zero at your next opportunity. If you drop it with a night scope on it, you might be writing Uncle Sam a fat check to buy a new night scope--it costs more than the rifle.
Dropping your weapon is not good. Don't do it.
Hope it helps.
HN