I believe that the THC will degrade with time and temperature to CBN. Data suggests that a 4 hour butter extract is about right for decarbing with water (100*C). Taking that out to 6 hours may do the trick. If you can find a chart with a longer time it may have CBN lines too.
One way I found was to forget about a jar that is exposed to ambient heat but not sunlight, for a year. The brown dry buds are little sleeping pills.
Edit: I was making a batch of cookies last night and thought of another opportunity to covert THC to CBN. Sometimes when I am in a hurry or lazy, I will melt the frozen cannabutter in a cup on the stovetop. Once it is melted I stir in the lechithin. Last night I got distracted and the butter sat there a while hotter than 210*F I imagine. This is another opportunity to tune your butter to make sleep effects. A good study showing the conversion would be better than guessing.