1. Great question. From a practical angle, I find that sorting Low to High, when applying frequencies in the morning or the day time, and then only sorting High to Low in the evening (with the intention of going to bed) allows for the greater chance you will be left refreshed during the day, yet relaxed and sleep-prone, when trending towards bed time.
In other words, the last trend your body experiences, in either scenario, yields the appropriate trajectory per your sleep habits. I don't find a difference in the efficacy of any particular set, if run in either direction, as long as the set is completed.
Otherwise, you may find that if you always break off a session early, and don't tend to complete the full set often, that you will be left with a deficit of a certain part of the range, limiting the effectiveness.
2.The basic logic is: as long as you run all the frequencies, regardless of the order, you should be OK. Dr Nenah Sylver, in her book of frequencies, she said that some frequencies (normally some pairs of frequencies) must run in sequence.
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