Several 'time zones' in the brain 

The purpose of the fMRI analyses was to quantify these observations by isolating the reactions in each region of the brain. In particular, it was possible to observe and compare the decline in responsiveness in certain regions of the brain during the wakeful phase (in blue in the illustration below). "We observed a fact that we were previously unaware of", Pierre Maquet is pleased to tell us. "Only the cortical areas reacted to increased sleep pressure." Observation of the fMRI also showed a drop in brain activity when melatonin was secreted. "And once again, the levels of responsiveness increased as soon as the melatonin was inhibited, even with increasing sleep pressure. Two factors that oppose each other, thus maintaining a certain level of performance."

"But the most surprising discovery in this study", the neurologist emphasises, "concerns the observation of a difference in the circadian phase between the six lobes that make up the cortex." During the experiment, the subjects' brain performance was recorded by fMRI 13 times. Thirteen points spread across 42 hours, arranged on a graph, showing a sinusoid dependent on the circadian rhythm. As the researchers were able to isolate the reactions in the different regions of the brain, it was possible to trace a sinusoid for each one of them. "I expected to find all of them at the same level. But although all of them did indeed evolve over a 24-hour cycle, they were misaligned. Some were ahead in relation to the melatonin peak, and others were behind. We recorded a difference of two hours between the responses of the regions most in advance and the regions that were most behind. This means that we don't have just one clock in our brain, but each region keeps its own time. It's a major discovery. The 'clock genes', which regulate biological activity over a 24-hour period are highly sensitive to the neurons' metabolic state. These misaligned sinusoids could mean that these genes adapt the activity phase of one region of the brain according to local energy needs, and therefore in function of neuronal activity."

It would therefore appear that we don't have one but several circadian clocks, all with a 24-hour cycle which, as well as aligning with the cycle of the sun, are regulated according to neuronal needs. By delving deeper into this study on gene transcription in the rat or mouse, it should be possible to better understand what causes some phases to be ahead of others. This research should help to better understand why some individuals are 'morning people' or 'evening people', or more sensitive to jet lag or night work, for instance. "This completely changes how we think of circadian clocks", the neurologist tells us enthusiastically. "Apparently, they are more flexible and more adaptable than we thought."    

Harnessing light 

While it would appear that the various regions of the brain have several 'time zones', there is nevertheless a master clock, located in the suprachiasmatic nucleus, which regulates neuronal activity during a 24-hour period. All the body's clocks are tuned to this one, which, in turn, is influenced by sunlight. For instance, it doesn't quite correspond to an Earth day, but naturally aligns with sunrise every morning. Another example: if you were to travel from Europe to San Francisco, the melatonin secretion cycle would adapt to the time change after several days of adjustment. Therefore, recognising the importance of the influence of the circadian rhythm on our performance and establishing the link with light through sleep studies, is one of the keys to significantly influencing economic change and public health. "Human errors at work or traffic accidents associated with nocturnal activity, and hence, states of somnolence, are considerable. And whatever we do, our biological clock works continuously and in sync with our planet. Fighting against it increases the risk of accidents, but also the chances of developing cardiovascular diseases such as high blood pressure, or diabetes. Disturbed sleep could even increase the risk of certain types of cancer. That's why shift work is disastrous. Our biological clock never has the time to adjust to a schedule." It is true that some jobs or leisure activities don't offer the possibility of sleeping from 22:30 to 06:30 every day. Using artificial light that diffuses the wavelengths imitating the behaviour of sunlight during the day could influence melatonin regulation and thus limit the effects of somnolence. "For instance, Derk-Jan Dijk, my colleague from Surrey, carried out such an experiment. He and his team changed the light bulbs in a company without informing the night shift teams, and they observed increased vigilance and performance at work." Initiatives such as these can also facilitate the lives of people with neurodegenerative diseases. Those who are affected by Alzheimer's, for instance, suffer significant neuronal loss in the suprachiasmatic nucleus, which disrupts melatonin secretion. This disruption leads to a state of psychomotor agitation at the end of the afternoon. "A syndrome known as sundowning, which is one of the reasons for placing the patient in an institution. By managing this phenomenon, especially through lighting, we could keep patients at home for longer." There is nothing new about using light to combat the damaging effects of somnolence. The technology is already on the market, but awareness needs to be raised to initiate a significant change in the workplace and recognise light as a factor that influences our sleep cycle.