Your Brain Is Bigger In The Morning

By Neuroskeptic | June 8, 2015 5:07 pm

The brain shrinks over the course of the day, ending up smaller in the evening – before returning to its full size the next morning. That’s according to a neat new study based on an analysis of almost 10,000 MRI scans. It’s published today in Neuroimage.

Kunio Nakamura and colleagues of the Montreal Neurological Institute examined 3,269 scans from multiple sclerosis trials and 6,114 from the ADNI Alzheimer’s disease project. This makes it the biggest neuroscience study I can think of.

The authors show that people’s brains are bigger when they’re scanned in the morning, compared to afternoon or evening scans of the same people.

Nakamura et al. defined brain size in terms of the brain parenchymal fraction (BPF), which is the proportion of the intracranial volume that’s filled with brain tissue. Essentially, the BPF is the degree to which your skull is full of brain.

The BPF fell by 0.18% over the course of the day in the multiple sclerosis studies, and by 0.44% in the Alzheimer’s dataset. Not a huge amount, but to put this in context, it’s roughly the same degree of shrinkage that someone with Alzheimer’s would experience over the course of a year.

Here’s an image showing BPF by time of day (8 is 8 AM, 18 is 6 PM) in the ADNI dataset:

brain_shrinkWhy does the brain shrink as the day goes on? Nakamura et al. suggest that fluids may be the key. Like a sponge, the brain gets bigger when it’s well hydrated:

A possible mechanism may be that lying down during the night is associated with a redistribution of body fluids that had accumulated in the lower extremities during the day… It is also possible that the effect of time-of-day is associated with hydration status.

The authors warn neuroscientists to control for time-of-day effects in future studies:

Our study suggests that a bias related to the acquisition time  exists, and this may be especially apparent in small studies where the time of image acquisition may not be fully random.

For example, a tendency to acquire MRIs from healthy subjects in the morning and the diseased group in the afternoon would bias the brain volumes towards a greater group difference in cross-sectional studies.

Neuroskeptic readers may remember that I made the same point a while back, after a paper showed changes in the functional properties of the brain as the day progresses.

ResearchBlogging.orgNakamura K, Brown RA, Narayanan S, Collins DL, Arnold DL, & Alzheimer’s Disease Neuroimaging Initiative (2015). Diurnal fluctuations in brain volume: Statistical analyses of MRI from large populations. NeuroImage PMID: 26049148

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    Hmmm. So do sleep deprived people not regain the shrinkage?

    • Skyler Webb

      Makes sense


    Also, fits with the “glia as nightly janitors” hypothesis (Science. 2013 Oct 18;342(6156):373-7. doi: 10.1126/science.1241224)

  • Sid

    does this have any impact on productivity, concentration etc…how does a common man make use of this study…

    • Neuroskeptic

      If it’s just a side effect of fluid levels, then it probably makes no direct difference.

      • SkepticalScientist

        How time of day interacts with cognition

    • Felonious Grammar

      Seems to me it’s just part of the rhythm of nature. Watch flowers in a flower bed long enough and you’ll see that their petals are lightly closing and opening as if they’re breathing. We tend to objectify living things, I think, and reduce or idealize them so that we think of them as static, when nothing could be further from the truth for the living.

      • Maia

        This human thought-habit is called reification: turning processes/flows into “things” that are unchanging. It warps our view of reality and leads us into mistaken actions of all sorts. Another one is either/or thinking… There are lots of these and they are incredibly hard for humans to keep in mind… even scientists.

    • Anonymouse

      You just take it as yet another thing that people doing imaging work should take into account, such that when two groups are compared and conclusions are drawn about, say, a cange in matter in their brains due to some presumably distinguishing factor, they aren’t systematically imaged at different times in their daily schedule.

      I can’t think of a good example for where this would be a likely confound (that’s good.), but that’s what I make of this, without expecting it to really have much of an influence on any important findings or any direct behavioural consequences.

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  • Overburdened_Planet

    This reminded me an article, “Getting Enough Sleep Can Be a Matter of Life and Death” in the April 2015 issue of Discover magazine.

    “In a 2012 paper, Nedergaard and her colleagues announced the discovery of a waste-disposal system in the brain, in which toxic metabolites such as beta amyloid — associated with Alzheimer’s and other neurodegenerative diseases — are flushed out by cerebrospinal fluid with the help of support cells called glia. In 2013, the team released a mouse study showing that this system turns on during sleep.

    “It’s been known for years that diseases like Alzheimer’s are associated with sleep disturbances,” Nedergaard says. “But maybe physicians should be treating sleep to slow the progression of the disease.” Indeed, she notes, a 2009 study at Washington University found that sleep-deprived mice developed Alzheimer’s-like brain plaques more often than their well-rested cousins. “Because it fits, right? If you don’t sleep, you don’t clean your brain.”

    Could there be a connection between these two observations?

  • Mona

    Could this be an explanation for the sundowning syndrome?

    • Simon Manning

      I suspect, like Neuroskeptic said in response to one of the other questions above, if the brain size differences is only a side effect of there being more or less fluid, then probably not. I think the leading theory on sundowning syndrome is that its caused by break down in the area of the brain that signals if you’re awake or asleep, leading your brain to think the latter is true.

  • hummingbird

    Water on the brain?

  • ontime2011

    What about the functioning of the brain more than hydration as a factor of shrinking?.The more use of and brain area the more flow of blood in it What about stress? The brain of a person that uses his cognitive capabilities shrinks more than the one of a person that develop simple brain tasks along the day?

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  • Disqourse

    This may explain why people with brain tumors commonly experience nausea and headache waking, and why the symptoms subside as the day progresses, only to return the next morning.

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No brain. No gain.

About Neuroskeptic

Neuroskeptic is a British neuroscientist who takes a skeptical look at his own field, and beyond. His blog offers a look at the latest developments in neuroscience, psychiatry and psychology through a critical lens.


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