Hot headed? Turns out a 40°C+ brain is normal!

New research shows that normal human brain temperatures vary much more than previously thought, and being warmer could actually be a sign of healthy brain function.

A study using magnetic resonance spectroscopy (MRS) produced the first 4D maps of healthy human brain temperatures. This map, dubbed HEATWAVE, shows changes in the human brain temperature through time, and highlights changes based on age, sex, menstrual cycles, and sleep cycles.

In 40 healthy adult volunteers (aged 20-40 years), oral temperatures were typically less than 37°C, but average brain temperatures tested through MRS were between 38.5°C, with deeper regions within the brain exceeding 40°C. This not only reveals that brains are warmer than previously thought, but challenges the widespread belief that human brain and body temperatures remain constant.

“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,” says Dr John O’Neill, group leader at the Medical Research Council (MRC) Laboratory for Molecular Biology, in Cambridge, UK. “Such high temperatures have been measured in people with brain injuries in the past, but had been assumed to result from the injury.”

Female brains on average were 0.4°C warmer than male brains, where females in the post-ovulation phase of their menstrual cycle were also 0.4°C warmer than females measured at the pre-ovulation phase. Our brain temperature also follows a sleep cycle, where our brains are the warmest in the afternoon, and lowest at night, varying by as much as 1°C.

“We found that brain temperature drops at night before you go to sleep and rises during the day,” says O’Neill. “There is good reason to believe this daily variation is associated with long-term brain health – something we hope to investigate next.”

Temperature also increased with age, where across the 20-year range of participants, there was an average increase of 0.6°C, particularly in the deep brain regions. This suggests that the capacity to cool down may deteriorate with age and could be linked to development of age-related brain disorders.

“Using the most comprehensive exploration to date of normal human brain temperature, we’ve established ‘HEATWAVE’ – a 4D temperature map of the brain,” says Dr Nina Rzechorzek, MRC Clinician Scientist Fellow. “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.”

The research, published in Brain, also shows that daily brain temperature cycles strongly correlate to the survival of patients with traumatic brain injury. The data collected from HEATWAVE could be used to advance the understanding, prognosis and treatment of brain disorders and injuries.

“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,” adds Rzechorzek.