Healthy human brains are hotter than previously thought, and can regularly exceed 40 degrees Celsius, a new study suggests.
The study also found that brain temperature drops at night, and increases during the day.
In healthy men and women, where the temperature in the mouth is typically less than 37C, average brain temperature is 38.5C.
However, deeper brain regions can often be hotter than 40C, particularly in women during the daytime.
While these temperatures would be considered a fever elsewhere in the body, researchers say they could be a sign of healthy brain function.
Dr John O’Neill, group leader at the Medical Research Council (MRC) Laboratory for Molecular Biology, said: “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.
“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.”
Previously, human brain temperature studies have relied upon data from brain-injured patients in intensive care, where direct brain monitoring is often needed.
But more recently, a brain scanning technique, called magnetic resonance spectroscopy (MRS), has enabled researchers to measure brain temperature non-invasively in healthy people.
Until now the method had not been used to explore how brain temperature varies throughout the day, or to consider how the body clock influences this.
The new study, led by researchers at the MRC Laboratory for Molecular Biology, in Cambridge, UK, has produced the first 4D map of healthy human brain temperature.
It overturns several previous assumptions and shows the extent to which brain temperature varies by brain region, age, sex, and time of day.
The researchers say their findings also challenge a widely held belief that human brain and body temperature are the same.
The research, published in the journal Brain, also included analysis of data from patients with traumatic brain injury, showing that the presence of daily brain temperature cycles strongly correlates with survival.
According to the study, these findings could be used to improve understanding, prognosis, and treatment of brain injury.
Researchers recruited 40 volunteers, aged 20-40 years, to be scanned in the morning, afternoon, and late evening over one day, at the Edinburgh Imaging Facility, Royal Infirmary of Edinburgh.
They also gave them 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.
Researchers found deeper brain structures were frequently warmer than 40C, with the highest observed brain temperature being 40.9C.
Across all of the people, brain temperature showed consistent time-of-day variation by nearly 1C, with highest brain temperatures observed in the afternoon, and the lowest at night.
Female brains were around 0.4°C warmer than male brains, the study found.
The scientists suggest this 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 0.4C warmer than that of females scanned in their pre-ovulation phase.
Brain temperature also increased with age over the 20-year range of the participants, most notably in deep brain regions, where the average increase was 0.6C.
The researchers 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
Dr Nina Rzechorzek, MRC Clinician Scientist Fellow from the MRC Laboratory for Molecular Biology, who led the study, said: “Using the most comprehensive exploration to date of normal human brain temperature, we’ve established ‘HEATWAVE’ – a 4D temperature map of the brain.
“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 TBI suggests that round-the-clock brain temperature measurement holds great clinical value.”
She added: “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.”