While the human body is normally around 37°C (98.6°F), the same can’t be said for the brain, it seems. Healthy human brains are hotter than previously thought and can be more than 2°C (3.6°F) warmer than the rest of our bodies, according to new research.
Healthy participants of a study, published in the journal Brain, had an average brain temperature of 38.5°C (101.3°F), a whole 2.5°C (4.5°F) hotter than the average oral temperature. In deeper brain regions, the temperature was found to often exceed 40°C (104°F), with 40.9°C (105.6°F) being the highest temperature recorded.
Brain temperature is not fixed, the study found – it varies more than scientists once thought, affected by age, sex, menstrual cycle, brain region, and time of day.
“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. Such high temperatures have been measured in people with brain injuries in the past, but had been assumed to result from the injury,” Dr John O’Neill, Group Leader at the Medical Research Council Laboratory for Molecular Biology, said in a statement.
The “normal” temperature of the brain has never actually been defined in humans. Instead, it is generally assumed to be the same as the rest of the body. Previous studies have used data from brain-injured patients, whose brains are directly monitored. Now, brain temperature can be measured in healthy people using magnetic resonance spectroscopy (MRS) – a non-invasive brain scanning technique.
Using MRS, the team behind the new study examined the brains of 40 healthy people – 20 men and 20 women – aged 20 to 40. Measurements were taken three times over the course of a day, making this the first time MRS has been used to track changes in brain temperature throughout the day.
Brain temperatures ranged between 36.1°C and 40.9°C (97°F and 105.6°F). The brain surface tended to be cooler, while deeper regions were found to be largely warmer. The thalamus, for example, which is one of the deepest parts of the brain, is where the highest temperature was recorded.
A person’s sex was also found to impact their brain temperature. Female brains were 0.36°C (0.65°F) warmer during the second half of the menstrual cycle, after ovulation, than they were during the first half or compared to the brains of males.
In all participants, brain temperature was found to vary by up to 1°C (1.8°F) throughout the day. Brains were hottest during the afternoon and coolest at night.
“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,” O’Neill added.
The researchers then created the first-ever 4D map of healthy human brain temperature. Applying this to patients with moderate to severe traumatic brain injury, they found that those without daily brain temperature rhythms were 21 times more likely to die in intensive care.
“Human brain temperature is higher and varies more than previously assumed,” the study concludes.
“This has major implications for temperature monitoring and management, with daily brain temperature rhythmicity emerging as one of the strongest single predictors of survival after brain injury.”
While the authors caution that their results are purely correlative at this point and need to be validated in larger studies, they remain optimistic that they could have clinical value in treating traumatic brain injury.
"Our work also opens a door for future research into whether disruption of daily brain temperature rhythms can be used as an early biomarker for several chronic brain disorders, including dementia,” lead author Dr Nina Rzechorzek, from the MRC Laboratory for Molecular Biology, said.