A study published in Brain shows that normal human brain temperature varies by region, age, sex, menstrual cycle, and time of day. The research team at the United Kingdom’s Medical Research Council (MRC) Laboratory for Molecular Biology created the first 4D map of a healthy human brain, including temperature.
The study included 114 study participants aged 20-40 years who had suffered from moderate to severe traumatic brain injury (TBI). They were scanned in the morning, afternoon, and late evening over the course of one day using a technique called magnetic resonance spectroscopy (MRS) that provided accurate brain temperature readings. The participants also wore a wrist-worn activity monitor in order to collect data that may shed light on genetic and lifestyle differences affecting the circadian rhythm. For example, readings were taken at specific wake-sleep cycle points to account for daily routines of “night owls” and “early birds.”
The patients’ average brain temperature was 38.5°C, but it varied from 36.1 to 40.9°C. The highest temperatures were observed in the thalamus. Healthy men and women have an average brain temperature of 101.3°F/38.5°C, with deeper brain regions often exceeding 102°F/40°C. Researchers found that brain temperature drops right before sleep and increases during the day. The brain surface was generally cooler while deeper brain structures frequently had temperatures over 40°C; with the highest observed brain temperature being 40.9°C. Brain temperature across participants showed consistent time-of-day variation by nearly 1°C, with highest brain temperatures observed in the afternoon, and the lowest at night.
Sex differences were observed. Female brains were roughly 0.4°C/34°F warmer than male brains, and researchers believe this difference can be attributed to the menstrual cycle. Most female participants were scanned in the post-ovulation phase of their cycle. At this phase, the women’s brain temperature averaged about 0.4°C warmer than temperatures of women participants scanned in their pre-ovulation phase.
There was sufficient data for 100 participants to test for daily rhythms, and only a quarter of these participants demonstrated a daily rhythm in brain temperature. When evaluating predictors of survival in intensive care, the researchers found that daily brain temperature variation was strongly linked with survival. Only 4% of TBI patients with a daily brain temperature rhythm died in intensive care compared to 27% of TBI patients who did not have a rhythm. The findings have implications for diagnostic assessment, survival prediction for TBI patients, and treatment of brain injury.
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 suspect that the brain’s capacity to lower and regulate brain temperature deteriorates with age. Further research is critical to determine if there is a link with the development of age-related brain disorders. The MCR research team will continue to research how daily brain temperature rhythm is associated with long-term brain health.