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Healthy human brains can be hotter than we thought, reaching nearly 41°C in women, according to a small study.

The findings could change how people with brain injuries are medically managed, says at the MRC Laboratory of Molecular Biology in Cambridge, UK. “This is going to give us another window into how the brain is working.”

Normal brain temperature is generally assumed to be the same as that of the rest of the body – about 37°C – but we had no way to know for sure. People with head injuries may have highly sensitive temperature probes put into their brains, but this isn’t done to people who don’t need their brain temperature monitored for medical reasons.

Rzechorzek’s team looked at the brains of 40 healthy people – half of whom were women – using a relatively new technique called magnetic resonance spectroscopy, which uses MRI machines to measure the temperature of different parts of the brain. This was the first time the technique has been used to measure brain temperature variation during the day and over the menstrual cycle. The study did not include transgender people.

The brain temperature ranged from 36.1°C to 40.9°C, with the average reading 2.5°C higher than the body temperature recorded in the mouth. This makes sense because the brain is highly metabolically active, says Rzechorzek.

The highest readings came from the thalamus, one of the deepest parts of the brain, which may be less cooled by the organ’s blood vessels. “It’s hotter in the core,” says Rzechorzek. The team also found that the brain is about 0.9°C cooler at night, which might be because there is greater blood flow to the organ when we sleep.

The brains of the women in the study were 0.4°C hotter during the second half of the menstrual cycle, between ovulation and menstruation, compared with the first half, and compared with men.

Doctors sometimes try to lower the body temperature of people with brain injuries because they are concerned that high temperatures are harmful, an approach that may need revising, says Rzechorzek. “We are making an assumption that a value of, for instance, 40°C is abnormal, but we don’t have any evidence to tell us that,” she says.