The human brain is big and has the capability of conjuring innovative solutions for the most complex problems. Inevitably, however, the brain shrinks with age as changes occur, from molecules to morphology, but these changes can also occur on a daily basis. According to a recent study published in the journal Neuroimage, the brain shrinks over the course the day, starting off bigger in the morning and ending up smaller at night. But why?

Similar to how bodies mature, the human brain tissue undergoes changes throughout life. Magnetic resonance imaging has allowed scientists from Stanford University to capture how the human brain tissue changes as we age. This helps doctors decipher what’s normal at different ages, and use this as a comparative analysis to tell if a person’s white matter — brain tissue containing nerve fibers that play a role in speed of nerve signaling — is out of the normal range based on the standard curve, according to a 2014 study published in the journal Nature Communications.

Nerve bundles in the brain tend to increase in volume until we turn 40 and then slowly start to deteriorate. By the end of our lives, the tissue is about the volume of a 7-year-old child.

Kunio Nakamura and his colleagues at the Montreal Neurological Institute sought to observe these fluctuations in brain volume in a single day to see when the brain is the biggest and the smallest. They analyzed over 3,000 MRI scans from multiple sclerosis trials and over 6,000 from the ADNI Alzheimer’s disease project in patients throughout the day. The effect of time of day on brain parenchymal fraction (BPF) would be compared among both groups.

In the study, brain size is defined in terms of BPF, which is the proportion of the intracranial volume that is filled with brain tissue. The BPF is the degree to which the brain fills the skull. The percentage change in BDF was modeled with a linear mixed effect (LME) model that was applied separately to the MS and ADNI datasets.

The findings revealed BPF changes were different for MS and Alzheimer’s patients. The BPF fell by 0.18 percent over the course of the day for the MS group compared to 0.44 percent in the Alzheimer’s group. According to Discover Magazine, this is roughly the same degree of shrinkage that someone with Alzheimer’s would experience within a year.

But why does the brain shrink throughout the day?

Nakamura and his colleagues believe it’s connected to fluids. The brain, comparable to a sponge, gets bigger when it’s 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," wrote the study authors.

The study did have some limitations when it came to the effect of time of day and brain size. For example, among the datasets used, acquisition time of the MRI may not have fully been random. The tendency to acquire MRIs from healthy subjects in the morning and the diseased group in the afternoon could have led to a greater group difference when it came to analyzing brain volumes. In other words, the time of day researchers conducted their experiments could have had an impact on the results when it came to the brain and MRI scans.

Changes in brain size are not just noticeable on a daily basis, however. They can also be seen during a woman’s menstrual cycle. A 2011 study published in the journal PLOS ONE found there are short-term alterations of the brain during menstruation. A significant gray matter volume peak, mostly composed of neuronal cell bodies, and cerebrospinal fluid (CSF), clear colorless liquid that cushions the brain within the skull, loss was seen at the time of ovulation in females. However, this volume peak was not related with estradiol or progesterone hormone levels.

There are also several genetic and environmental factors that can influence the size of our brain.

Sources: Arnold DL, Brown RA, Collins DL et al. Diurnal fluctuations in brain volume: Statistical analyses of MRI from large populations. Neuroimage. 2015.

Mezer AA, Wandell BA, and Yeatman JD. Lifespan maturation and degeneration of human brain white matter. Nature Communications. 2014.

Fitzek C, Gaser C, Hagemann G et al. Changes in Brain Size during the Menstrual Cycle. PLoS ONE. 2011.