Excellent question, Vader. My short answer first ... consider individual variation.
Now let's look at some generalities. Mercola has a good article on exercise and cognition at http://fitness.merco...s-exercise.aspx, and has some good pieces specifically on HIIT. The Wikipedia article on the "Neurobiological effects of physical exercise" - https://en.wikipedia...ysical_exercise - is excellent, covering a wide spectrum of exercise types and cognitive benefits at all ages, addressing gross effects as well as specific mechanisms. It cites scores of references for further information. I think it gives a very accurate picture of what the average person can expect. But that is the nature of statistics. They can only speak to groups and averages and don't tell you exactly what you as an individual will experience ... just a tendency.
My experience bears this out. I do burst training. It's sort of like HIIT except with just one intense interval per session. Most mornings I do about a one minute full on sprint down the street, then walk back. The whole thing takes maybe five minutes or so but gets my heart pumping and breathing elevated for about a half hour. The immediate benefits last all day, and the cumulative benefits of course much longer. I do resistance exercise a couple times a week for strength and cycle for enjoyment. Besides being effective, I like it because I'm time constrained and can pack a lot of punch into a short period of time. If it works for me, it will work for some other people too ... but not necessarily everybody. For example I'm in my late fifties and have parameters that would be different than twenty-something's. Even other people my age though could easily have quite different experiences.
One thing that virtually everybody has in common though is a biphasic (inverted U) response curve. From no exercise at all, the benefit increases as you increase the intensity and/or the amount of time spent each week exercising. At some point the maximum benefit is reached and from there further increases bring diminishing results. This is as much common sense as anything else ... take as an elementary example water consumption. If you get no water, you will die. From that point, increasing the amount of water you drink brings increasing survival and health. At some point, maximum benefit is reached, and more water brings declining benefits. If you try to drink ten gallons at a sitting, for example, you may die. This may seem even too commonsense to be meaningful, but if you think about it it's a very general principle that provides a useful context in which to evaluate the health benefits of a vast spectrum of things. Once explicitly recognized, it becomes a matter of establishing where a given level is at on the response curve to speculate whether more or less would be fruitful.
Of course there are other variables, too. My personal opinion would be that any strenuous four hour workout would be much too much for me. Even ninety minutes a day would be too much, if not because of my intrinsic response curve then because of the need to balance other demands on my time. But that's just me. As always, your mileage may vary.
Cheers!
Edited by Kinesis, 05 May 2016 - 06:03 PM.
