The equivalent temperature of a chip in a smartphone using a 4G network.
We know that body temperature, also known as core temperature, is currently measured most accurately at 36.77 degrees Celsius. This figure is significantly lower than the 37 degrees Celsius benchmark that scientists have recorded since the 19th century.
Part of the reason is that modern temperature measurement technologies have achieved much higher resolution compared to the mercury thermometers used two centuries ago. But scientists also indicate that human body temperature is gradually cooling down—by 0.03 degrees Celsius every decade.
This means that boys born from the year 2000 onwards are 0.59 degrees Celsius cooler than their great-great-grandparents, who were born in the early 1800s. Moreover, the trend of decreasing body temperature is more pronounced in women than in men.
However, there is a contrasting trend; in a new study published in the journal Brain, scientists report that while women have lower body temperatures than men, their brains are actually warmer.
Similar to a computer CPU, the brain continuously generates heat during its operation. This new study shows that some deep parts of the brain can reach temperatures close to 41 degrees Celsius. On average, women’s brains are 0.4 degrees Celsius warmer than men’s.
These figures also surprised scientists, as they previously did not think our brains could get that hot.
John O’Neill, the study’s author and a biologist at the UK Medical Research Council’s Molecular Laboratory, stated: “For me, the most surprising finding from this research is that a healthy person’s brain can reach temperatures diagnosed as fever anywhere else in the body.”
Previous studies have recorded brain temperatures up to 41 degrees Celsius, but only in patients with brain injuries, certain neurological disorders like Parkinson’s, or those with cancerous tumors.
However, in this new study, scientists indicate that this high temperature is not a sign of malfunction but may actually be evidence that the brain is functioning normally and healthily.
Particularly, the new technique they used to measure brain temperature could be applied in the future to identify signs of damage or disorders in patients, ranging from headaches, epilepsy, multiple sclerosis to strokes, traumatic brain injuries, and cancer.
How is brain temperature measured?
When you have a fever, you take your temperature in your armpit, groin, or mouth. There are several electronic thermometers commonly used during the COVID-19 pandemic that can measure temperature by firing infrared rays onto your skin, such as on your forehead. However, all these approaches fail when it comes to measuring brain temperature.
This is because the brain is protected by the skull, which infrared rays cannot penetrate. You also cannot use contact thermometers to measure the temperature of a healthy person’s brain.
Although in some cases—typically during brain surgeries—scientists have done this. They drill a hole in the skull, insert a thermometer into the part of the brain that needs measuring, and record the contact temperature there.
Unfortunately, for healthy individuals, this invasive procedure is very risky. It can cause damage to the brain and leave permanent scars. Meanwhile, the reward is often just a number that is sometimes meaningless.
To develop a non-invasive method for measuring brain temperature, scientists had an idea in the past. They injected fluorescent substances into the brains of mice and then shone near-infrared light (the highest energy type in the infrared spectrum that can penetrate the skull) onto them.
Because the thermosensitive fluorescent substances can emit different wavelengths when stimulated by near-infrared light, scientists can use a super-sensitive sensor to capture those wavelengths.
The data is then reconstructed to form a heat map of the brain. The advantage of this method is that it allows for real-time temperature measurements in the brain. However, it was only conducted by directly injecting the fluorescent substance into the mouse’s brain.
This is strictly prohibited in humans, as injecting fluorescent substances directly into the brain can be dangerous. If injected into normal blood vessels, the fluorescent substance cannot cross the blood-brain barrier—a protective barrier for the brain, with proteins acting as “gatekeepers” allowing only nutrients to pass through while blocking all components deemed harmful to the brain, including bacteria, viruses, chemicals, and drugs.
In summary, the approach using fluorescent substances still fails to measure temperature in the human brain. Therefore, in their research, the scientists at the MRC used a new method.
They call it magnetic resonance spectroscopy (MRS). Instead of using infrared light to stimulate fluorescent substances, scientists directly send electromagnetic waves into the brain and record the variations of these waves as they interact with heated tissues and reflect back.
The result will produce a 4D heat map, capturing the temperature of different brain regions in real-time with relatively high resolution.
Scientists discover our brains can reach 40 degrees Celsius during the day
To test the effectiveness of the MRS technique, scientists selected 40 healthy volunteers aged 20 to 40 and measured their brain temperatures at three different times of the day: morning, afternoon, and night.
The results showed that our brains are up to 2 degrees Celsius hotter than the rest of our bodies. Specifically, the average temperature of the human brain is 38.5 degrees Celsius, higher than the 36.5 degrees Celsius measured under the tongue.
During the day, brain temperatures are higher than at night when volunteers are preparing to sleep. O’Neill stated, “There is good reason to believe that this daily variation is related to the long-term health of the brain.”
Additionally, women’s brains are warmer than men’s by an average of 0.4 degrees Celsius, likely due to their menstrual cycles. Notably, the highest recorded temperature in a healthy brain reached 40.9 degrees Celsius, equivalent to the temperature of a chip in a smartphone using a 4G network.
In comparison, patients with traumatic brain injuries can push their brain temperatures up to 42.3 degrees Celsius. Scientists are investigating the relationship between brain temperature and their survival rates.
Nina Rzechorzek, a neuroscientist from the MRC Molecular Biology Laboratory, noted that measuring brain temperature in real-time has significant clinical implications. This could allow doctors to monitor this metric around the clock, similar to a patient’s heart rate or blood pressure.
Brain temperature provides essential information for assessing patient health. At the same time, it could serve as a biomarker for diagnosing potential future brain disorder risks.
The magnetic resonance spectroscopy technique promises to be applied in many hospitals, enabling doctors to care for patients with epilepsy, multiple sclerosis, and those recovering from strokes, traumatic brain injuries, and cancer.
