The heat of summer can also be another reason to fish at night. During the day in the summer heat, water temps can reach a point where the trout's metabolisms slow down so much that they stop feeding. It may not be until late in the evening - perhaps during a summer hatch - when any trout begin to feed at all. On popular streams where this happens it means you'll have many other anglers on the water with you in the evenings. Perhaps it would be best to wait until they all went home?
I've done a bit of nocturnal fly fishing myself & can tell you that it can be an exciting & frustrating game. I'm still learning that game (boy, am I still learning), but there are a few things I've learned mostly the hard way:
-Don't ever fish any water that you haven't fished before in the daylight. The more you know the stretch of stream you're on at night, the better. You could get very hurt - or worse - if you go parading around in the dark on unfamiliar water. Always tell someone where you'll be fishing or fish with a friend.
You also need to know the lay of the land for casting as well, or you'll spend most of your time in the trees & bushes instead of the water. Also, knowing where to wade & walk will help you know that you're in the right position for casting to where the fish are, or at least to where they were in the daylight hours.
-Bring along a couple of flashlights: a small one for tieing on flies, changing tippets, etc & a larger one for getting to & from the stream. DO NOT shine the light on the water. You'll put down every fish for quite a ways.
-It's best to fish when the moon is dark. Fishing during a bright full moon can be almost like fishing in daylight. The fish can see you very well. If you are fishing on a night with a lot of moonlight, hit the shady spots.
-Standard protocol for night fly fishing is to use a fly that puts a lot of vibration in the water - a palmered marabou or deer hair-headed fly. I've found this to be true, but you can also fish wets, nymphs, or dries as well.
Nymphs: You want to get down on the bottom, with your fly bouncing along at the speed of the water current. Get a good, natural drift with no drag on your line. This could very well mean fishing upstream.
Wets: Down & across seems to have always worked well for me. I like to fish these just below the surface. Also, you can get away with using a much larger wet fly than normal, 2 to 4 hook sizes bigger than you might during the day. I've also found a black colored fly works better for me than any other color. I don't know why this is, except maybe the trout can see the fly's silhouette better.
Dries: I've fished dry flies at night, but unless there's some really big hatch activity going on in the middle of the night, like a spinner fall, I don't usually do that good with them. However, I have found that you can get away with some drag on your dry fly at night. A little movement to your dry fly, to imitate a fluttering spinner, actually seems to help. Maybe it draws the trout's attention?
-Night time fly fishing, no matter what time of year, is a slow game. The fish seem to slowly suck in your fly rather than lunge at them. At night trout seem to be feeding either on the bottom or just below the surface, too unless a large amount of insect activity is on the surface.
-Because it is slow at night, I try to slow down myself. I try to get really methodical & hit every spot in a pool or run many times. I'll work each spot over & over & in different ways, working my way through a run. Then I take a few minutes break & do it all over again - three times or more. Sometimes you pick up a good fish in a spot you had tried before.
-Water temp is just as important at night as it is in the day. Know the water temp where you're fishing. You need to find the water that is best for trout to be feeding in. This goes for all times of the year.
-Pick out spots you know have trout in them. You saw them during the daylight hours or did good there during the day time. Learn that spot very well. That's where to go at night.
Just remember that nocturnal fly fishing is a slow game. You won't always catch fish. Be patient & methodical. It's okay to just leave your nymph or streamer on the bottom for a while, not moving. When you do move them, try to get your flies to move at the speed of the current. You're going to miss some strikes for sure, & it will be frustrating at times, not being able to see as well as during the day. Still, it can be some of the best, most amazing fly fishing you'll ever do. Besides, what else would you be doing at 3 am, sleeping?!!?