This particular method is simple: sleep. Eight hours usually does it for me, >90% of the time the anger originated on the previous day. For others for whom this works, I expect the amount needed will vary from person to person.
Relevant medical research comes from many sources I have read, speculating on the (still unknown) complete list of the purposes of sleep, which includes (in lay terms) “washing away” junk the brain doesn’t need any longer. I am of the opinion that anger qualifies for that category.
My evidence: repeatedly observing this happening to me, hundreds of times.
Replication of experiments, and creation of new ones, to search for more evidence, is obviously needed. While this is a testable hypothesis, I certainly have not conducted a definitive test. For one thing, this lies outside the fields I have studied, formally, the most, and my sample size (one) was far too small to count for much.
An important point, in case anyone is wondering: no, I do not think this ability is limited to any one segment of the population, such as those with Asperger’s. If “Aspies,” like myself, have any advantage at all in this area, it’s limited (in my opinion) to the fact that many of us spend an unusually-high amount of time studying our own minds, and how they work. However, my hypothesis does not require that one know what the hypothesis states, which is no more than this: in a majority of the human population, the activity of sleep reduces levels of anger. Clearly, more reliable results could best be obtained by double-blind studies.
If I’m right, chronically sleep-deprived people, as a consequence, will be more likely to be angrier, on average, than is the case, overall, in the general population. This offers another avenue for testing.
Comments are welcome, especially regarding other research on this subject.
Also, please comment if you know of a good method for anger-elimination, or anger-reduction, which does not require sleep — for I may wish to try it myself.