The Human Brain Runs Way Hotter Than We Ever Realized

Mechanical systems, from your car's engine to the components in your laptop, heat up when they work harder. New study has discovered that the brain is the same way – and that it operates hotter than previously assumed.

According to a new study, some areas of the deep brain may reach temperatures of up to 40 degrees Celsius (104 degrees Fahrenheit), albeit this varies depending on sex, time of day, and other factors. In comparison, the usual mouth temperature in humans is often less than 37 °C (98.6 °F).

Researchers believe that this isn't an indication of dysfunction, but rather proof that the brain is functioning normally. In the future, unusual heat signatures might be utilized to check for evidence of brain injury or dysfunction.

"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 scientist John O'Neill of the Medical Research Council (MRC) Laboratory of Molecular Biology in the UK.

"Such high temperatures have been measured in people with brain injuries in the past, but had been assumed to result from the injury."

Previously, scientists' only knowledge into brain temperatures came from data gathered from patients with brain damage, which isn't the same as recording the condition of the brain in regular life.

The researchers utilized a technique called magnetic resonance spectroscopy (MRS), which involves sensing chemical patterns using magnetic fields, to non-invasively quantify brain temperature in 40 healthy volunteers ranging in age from 20 to 40 years old. They also merged this data with information on circadian rhythms and time of day.

The researchers discovered that the average brain temperature was 38.5 °C (101.3 °F), which was more than 2 degrees higher than the temperature beneath the tongue. The time of day, the brain area, and the participant's age, sex, and menstrual cycle all had an impact on the results.

Female brains were 0.4°C (0.72° F) warmer on average than male brains, most likely due to the menstrual cycle, with the maximum brain temperature measured at 40.9 °C (105.6 °C). The outer portions of the brain were usually colder, with daily changes averaging roughly 1 °C (1.8 °F).

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

The researchers were able to create a 4D temperature map of the brain as a result of their study. They want to use it as a reference tool for what a healthy brain should look like, albeit considerably more data from a much bigger group of people is required to make it truly helpful.

Further testing on 114 persons who had had a traumatic brain injury (TBI) revealed that the temperature of their brains varied even further, from 32.6 to 42.3 degrees Celsius (90.7 to 108.1 degrees Fahrenheit).

Although additional study is needed, there appears to be a relationship between brain temperature rhythm and the risk of surviving a TBI.

When you add in the fact that brain temperature appears to rise with age, it appears that controlling this temperature is crucial to maintaining a healthy brain – and surviving the effects of a serious brain damage. Temperature rhythms might potentially be used as indicators to assess the likelihood of developing future brain diseases.

"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,"  says MRC Laboratory for Molecular Biology neurologist Nina Rzechorzek.