Brain perfusion, as referred in this article, includes delivery of nutrients and removal of wastes, besides the usual non-stop gas transfer. From my observations on REM sleep (or the deprivation of REM sleep to be exact) and taichi (the extra slow moving type which can trick the brain that he/she is not moving to simulate REM sleep), I could only say that brain perfusion happens probably not more than 30 minutes for a 7-hour good sleep. For city folks who are under constant stress, they will get less because they would have less REM sleep. During day time, the situation gets worse if they don’t do enough short walks or equivalent exercises (Note: endurance exercises are not counted at all). I believe that many chronic diseases including Alzheimer’s, HIV, cancers, etc. would be difficult to heal once the patients become immobile and/or cannot sleep well. Their brain cells would deteriorate slowly due to malnutrition and waste accumulation. Downstream organs will also be affected slowly one by one.
Article “Rapid Eye Movement In REM Sleep Is Slow Motion Taichi ” - T...
For starters, neurons require an almost non-stop supply of glucose (the brain's main source of energy) or they will stop functioning efficiently or die, as they have very limited internal stores of glycogen. This is supplied via the blood, which constantly delivers glucose to the brain - if the blood contains too little glucose (such as during hypoglycemia in diabetics using insulin), symptoms like confusion and hypoglycemia start to appear very soon. This alone is a decent indicator that 'perfusion' as you define it happens constantly and not just thirty minutes per sleep cycle.
You can also map glucose consumption using FDG-PET: 18F-FDG is injected into the bloodstream, gets transferred into cells in the brain, and gets phosphorylated just like normal glucose would during metabolism. Again, the fact that this radioactive isotope passes the blood-brain barrier constantly suggests that nutrient delivery is a continuous process, and the fact that it gets phosphorylated suggests that there is metabolism going on.
Furthermore, your definition of 'perfusion' is incorrect, as perfusion simply refers to the delivery of blood to an organ.
Finally, I'd be interested to see some proof that nutrient delivery and waste removal does *not* happen continuously, because this is the first time I've heard of it. From where did you get this 30 minute number? Increased heart rate does not necessarily imply increased brain perfusion, by the way, as cerebral blood vessels may constrict. I would suggest having a look at the literature on these subjects. I do agree that regular excercise and moving away from your desk every now and then will do your brain a lot of good, though!
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3900881/
https://en.wikipedia.org/wiki/Fludeoxyglucose_(18F)
In addition to Kristof's remarks above, I would add some comments about waste removal. You may be interested in doing a search and learning about the waste removal system in the brain that researchers have called the glymphatic system. Recent research has also revealed that the brains lymphatic system in mammals may be more extensive than once thought (see July 2015 article in Nature). I think your thoughts about poor sleep having ramifications for dementia and other diseases will in the future prove accurate - several of the toxins found to be involved in glymphatic waste removal are also implicated in dementias.
I'd like to put it simple: a brain which isn't perfused for 30 minutes is dead. The proof of circulatory arrest is one of the supplementary criteria for brain death.
So maybe the question is rather about local autoregulation or regulation as needed?
Thanks for all the responses so far. But what I want is proof (in vivo). Deprivation of oxygen will cause brain death very fast, but brain cells won't die until possibly hours later. My assumption is that gas transfer happens all the time but not nutrients and wastes, and that I don't call it a full brain perfusion. Deprivation of glucose is probably less severe in this sense. The question is how much of glucose is required? Also, do brain cells take turns to work and rest so that all the brain functions can last longer? Back to my theory, a normal human being still earns a few tens of seconds/hour of full brain perfusion (i.e. during a pause in a physical action), unless the person is bed-ridden or exercising non-stop. Is that enough? If so, that could be the reason why people has assumed all the while that brain perfusion happens every minute.
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