Scientists have established a 4D map of human brain temperature and shown how this parameter varies with time of day, brain region, age, and sex in adults. They’ve found that in healthy men and women, where oral temperature is typically less than 37 degrees Celsius, average brain temperature is 38.5 degrees Celsius, with deeper brain regions often exceeding 40 degrees Celsius, particularly in women during the daytime.
Healthy brain temperature varies by time of day. Image credit: Rzechorzek et al., doi: 10.1093/brain/awab466.
“Abnormal temperature has been recognized as a sign of disease for more than two millennia,” said Dr. John O’Neill, a researcher at the MRC Laboratory for Molecular Biology, and colleagues.
“Both the temporal and spatial dynamics of temperature contain additional diagnostic information, exemplified by disrupted circadian rhythms, and local warming at sites of injury or infection.”
“Brain temperature is rarely measured directly since invasive methods are required; in practice, it is assumed to match the body core, overlooking the clinical importance of brain-specific measurements.”
In the new study, the researchers recruited 40 volunteers (20 males, 20 females, 20-40 years) to be scanned in the morning, afternoon, and late evening over one day.
Crucially, they also gave the participants a wrist-worn activity monitor, allowing genetic and lifestyle differences in the timing of each person’s body clock, or circadian rhythm, to be taken into account.
For both ‘night owls’ or ‘morning larks,’ knowing the biological time-of-day that each brain temperature measurement was taken at allowed differences between each volunteer’s body clock to be factored into the analysis.
In healthy participants, the average brain temperature was 38.5 degrees Celsius, more than two degrees warmer than that measured under the tongue.
The authors also found that brain temperature varied depending on time of day, brain region, sex and menstrual cycle, and age.
While the brain surface was generally cooler, deeper brain structures were frequently warmer than 40 degrees Celsius; with the highest observed brain temperature being 40.9 degrees Celsius.
Across all individuals, brain temperature showed consistent time-of-day variation by nearly 1 degrees Celsius, with highest brain temperatures observed in the afternoon, and the lowest at night.
On average, female brains were around 0.4 degrees Celsius warmer than male brains.
This sex difference was most likely driven by the menstrual cycle, since most females were scanned in the post-ovulation phase of their cycle, and their brain temperature was around 0.4 degrees Celsius warmer than that of females scanned in their pre-ovulation phase.
The results also showed that brain temperature increased with age over the 20-year range of the participants, most notably in deep brain regions, where the average increase was 0.6 degrees Celsius.
The scientists propose that the brain’s capacity to cool down may deteriorate with age and further work is needed to investigate whether there is linked with the development of age-related brain disorders.
“To me, the most surprising finding from our study is that the healthy human brain can reach temperatures that would be diagnosed as fever anywhere else in the body,” Dr. O’Neill said.
“Such high temperatures have been measured in people with brain injuries in the past, but had been assumed to result from the injury.”
“We found that brain temperature drops at night before you go to sleep and rises during the day.”
“There is good reason to believe this daily variation is associated with long-term brain health — something we hope to investigate next.”
To explore the clinical implications of data obtained from healthy volunteers, the team analyzed temperature data collected continuously from the brain in 114 patients who had suffered from moderate to severe traumatic brain injury.
The patients’ average brain temperature was 38.5 degrees Celsius, but it varied even more widely, from 32.6 to 42.3 degrees Celsius.
Of 100 patients for whom there was enough data to test for daily rhythms, only a quarter had a daily rhythm in brain temperature.
“Using the most comprehensive exploration to date of normal human brain temperature, we’ve established ‘HEATWAVE’ — a 4D temperature map of the brain,” said Dr. Nina Rzechorzek, a researcher at the MRC Laboratory for Molecular Biology.
“This map provides an urgently-needed reference resource against which patient data can be compared, and could transform our understanding of how the brain works.”
“That a daily brain temperature rhythm correlates so strongly with survival after traumatic brain injury suggests that round-the-clock brain temperature measurement holds great clinical value.”
The study was published in the journal Brain.
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Nina M. Rzechorzek et al. A daily temperature rhythm in the human brain predicts survival after brain injury. Brain, published online June 13, 2022; doi: 10.1093/brain/awab466
