I can't vouch for the results reported about yet another brain research experiment. I'm sure this one has some faults too, but it does seem to have made some good observations about our success in trying to recall words. The report was here at Science Daily.
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.
As I have previously stated, I believe the characters (or words) you are wanting to learn can't just be left alone until the next time they show up in your SRS review. When you want to purposefully learn something, it is best that you are seeing it every day. A review in an SRS has nothing associated with it. It's not as effective as when the word keeps coming up in something you are reading. And if you are doing lots of reading, you are getting reviews of all the words you need to learn. Whatever doesn't come up in your reading, you don't need to learn. If you think it really is something you need to learn even though you haven't seen it before, then you're reading the wrong materials.
By the way, I'm no expert, nor even a good example. I need to do a lot of reading too. Whenever I tilt my head down to read, I think it cuts off oxygen to my brain. I often get this heavy feeling in my head. It's better to keep your head up normally, but I can't hold a book up for that long either. Another problem is, if I'm reading something uninteresting, I get bored and just can't continue on.
Hopefully, reading will get easier for me soon.