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Comment by seesawtron | Hacker News Reader

It is to be noted that they found "evening types" people had greater grey matter volume in an area of the brain called the precuneus, NOT in the whole brain.

Also it is not clear whether more grey matter makes the individuals "evening types" or the other way around. This study does not discuss that. So jumping to theories about personality traits seems quite far fetched.

Evening types were younger than morning types in their sample of studied people. It is known that there is a decrease in grey matter volume between adulthood and old age, whereas white matter volume was found to increase from age 19-40, and decline after this age (Bartzokis et al.). So it is not surprising that their data showed evening types (younger people) had higher grey matter than morning types (older people).

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All of this seems fair, so an interesting note off of this:

I find the causality/correlative arrow here interesting as no matter the direction, there is interesting insight.

That would be interesting but I am not sure if this study is solid to even show existence of this arrow. That assumes that there are no confounding variables but just a one directional arrow either way. However several confounding variables likely exist (maybe age, maybe more things we haven't thought about) that could give rise to reduction in gray matter, change in sleeping patterns, so on and so forth.

Since this is not a behavioral study where experimentalists can actually control for confounding factors (like in animal studies), this correlation/causation arrow likely is a spurious observable effect. An ideal experiment would involve studying say lab animals in a controlled setting where "everything else being equal" one group sleeps during day and other during night and then observe their gray matter. Perhaps similar studies already exist but this paper doesn't refer to anything.

Mean age for morning types was ~56, for evening types it was ~54. Seems unlikely that changing grey matter volume with age is explanatory here.

If mean age between MT and ET can be significantly different ( p<0.001), is it unimaginable that the these two groups would show significantly different gray matter too?

Takeuchi H, Taki Y, Sekiguchi A, et al. Regional gray matter density is associated with morningness-eveningness: Evidence from voxel-based morphometry. Neuroimage. 2015;117:294-304. doi:10.1016/j.neuroimage.2015.05.037 from https://pubmed.ncbi.nlm.nih.gov/26003859/

> 432 healthy men and 344 healthy women (age, 20.7±1.8years)

> We demonstrated that morningness (less eveningness) was associated with (a) lower regional gray matter density (rGMD) in the precuneus and adjacent areas, (b) lower rGMD in the left posterior parietal cortex and adjacent areas, and (c) higher rGMD in the bilateral orbitofrontal cortex. Further, our exploratory analyses revealed that (d) higher rGMD in hypothalamic areas around the bilateral suprachiasmatic nuclei were associated with morningness.

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