Healthy human brains are hotter than previously thought, and can regularly exceed 40 degrees Celsius, a UK study suggests.
The study also found that brain temperature drops at night, and increases during the day.
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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 day.
Researchers say these temperatures could be a sign of healthy brain function.
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The study was led the Medical Research Council Laboratory for Molecular Biology, in Cambridge, Britain,
Group leader John O'Neill said the most surprising finding was that the healthy human brain could 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," he said.
"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 on data from brain-injured patients in intensive care, where direct brain monitoring is often needed.
But more recently, a technique called magnetic resonance spectroscopy has enabled researchers to measure brain temperature non-invasively in healthy people.
The study overturns several previous assumptions and shows the extent to which brain temperature varies by brain region, age, sex and time of day.
Scientists 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 includes analysis of data from patients with traumatic brain injury, showing the presence of daily brain temperature cycles strongly correlates with survival.
These findings could be used to improve understanding, prognosis and treatment of brain injury.
