The biggest brown trout have always been nocturnal. And eventually a fly fisherman enters his big fish phase, after he exits the catch as many fish as I can phase. For a short time, the fly man will beat the waters in search of his monster during the day, until he realizes that he can not seem to get a fish over the 18 inch mark on any of his creations while the sun is still out.
On Western streams, big browns are not exactly hard to come by, but in the more earthly locations that most people are forced to fish, slob browns are very difficult to come by. A fish breaking 20 inches is quite an accomplishment, especially on a fly. So while a wooly bugger on a size 4 hook might land a 20 incher in the west, in most places with poor water quality and tons of fishing pressure, big trout look for considerably bigger meals, and to catch them, you need to beef up your offerings considerably.
In small, deep streams that get a decent amount of fishing pressure, night fishing with big flies is often the only way you are going to see a fish over 18 inches. Letting a fly sink is often impossible in snag infested streams at night. The only option is to get a fly that rides the surface or slightly below the surface. A hand tied fly on a size 1 or 2 hook is perfect for the job. Considering you will be night fishing, there is no need to get pretty with the tying. Simply tying a gob of deer hair near the eye of the hook to create a wing effect that floats on or slightly under the surface is all you need to make an amazing night brown trout fly