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Basically, I think it says that you have better recall ability when the theta waves in the brain are high.
To me, this relates to SRS, because I have questioned the effectiveness of Space Repetition. Let's say you're using an SRS to try not to forget the Kanji. If you do your SRS review while the theta waves in your brain are low, your results will not be as good as if you had used the system during high theta wave activity. This will affect the SRS's ability to determine the next optimal time to show you the ones you are failing. So, say you have a bad day and get a lot of them wrong. Now there will be a day in the future where your need-to-review words will be higher.