A new study links a daily cycle in brain temperature with survival after traumatic brain injury.

New research has shown that normal human brain temperature varies much more than we thought, and this could be a sign of healthy brain function. In healthy men and women, where the oral temperature is typically less than 37°C, the average brain temperature is 38.5°C, with deeper brain regions often exceeding 40°C, particularly in women during the daytime.

Previously, human brain temperature studies have relied upon data captured from brain-injured patients in intensive care, where direct brain monitoring is often needed. 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, however, MRS had not been used to explore how brain temperature varies throughout the day, or to consider how an individual's 'body clock' influences this. The new study, led by researchers at the Medical Research Council (MRC) Laboratory for Molecular Biology, in Cambridge, UK, has produced the first 4D map of healthy human brain temperature.

This map overturns several previous assumptions and shows the remarkable extent to which brain temperature varies by brain region, age, sex, and time of day. Importantly, these findings also challenge a widely held belief that the human brain and body temperature are the same.

The research, published in the journal Brain, also included an analysis of data from patients with traumatic brain injury, showing that 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.

Striking brain temperature variation in health. To study the healthy brain, the 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.

Crucially, they also gave the participants 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.

For both 'night owls' and 'morning larks', knowing the biological time-of-day that each brain temperature measurement was taken at allowed differences between each volunteer's body clock to be factored into the analysis.

In healthy participants, the average brain temperature was 38.5°C, more than two degrees warmer than that measured under the tongue. The study also found that brain temperature varied depending on: time of day, brain region, sex and menstrual cycle, and age.

While the brain surface was generally cooler, deeper brain structures were frequently warmer than 40°C; with the highest observed brain temperature being 40.9°C. Across all individuals, brain temperature showed consistent time-of-day variation by nearly 1°C, with the highest brain temperatures observed in the afternoon, and the lowest at night.

On average, female brains were around 0.4°C warmer than male brains. This sex 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 0.4°C warmer than that of females scanned in their pre-ovulation phase.

The results also showed that brain temperature increased with age over the 20-year range of the participants, most notably in deep brain regions, where the average increase was 0.6°C.  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.