Written by : Philippe Lecrenier
Based on the work of:
How do sleep deprivation and our biological clock influence the development of our cognitive performance when we stay awake? Understanding these mechanisms is a major public health issue, because our ability to perform is significantly affected by disrupted sleep. Many work accidents and numerous diseases are linked to it and yet it is difficult to quantify the influence of jet lag, shift work – especially night shifts, or just everyday habits that don't respect the natural cycle. A study published in Science(1) helps us to better understand the mysteries of these complex mechanisms and their influence on our brain. With the help of functional magnetic resonance imaging (fMRI) and the execution of tasks repeated during a 42-hour period of wakefulness, researchers were able to observe the evolution in performance and brain responses to sleep deprivation and the circadian cycle in 33 subjects. Better still, they managed to map the incidence of each of these mechanisms, and detect not just one biological clock, but several, adjusted to different times according to the region of the brain. Something of a holy grail for sleep studies in general.
As long as we remain awake, we interact with our environment. However, this faculty is maintained on a continuous basis. Our cognitive performance and our ability to perform tasks evolve during the day and night. Daily variations that are influenced by two determining factors: our biological clock on the one hand, which follows a circadian rhythm (a period of 24 hours and 10 minutes to be exact), and sleep debt on the other hand, which expresses the accumulation of sleep pressure due to neuronal activity. "This biological clock is present in all living beings on our planet", explains Pierre Maquet, professor at the University of Liège, head of the Neurology Department and a researcher at GIGA-CRC in vivo Imaging. "It is naturally linked to the time it takes the Earth to rotate on its axis allowing life to adapt to it as optimally as possible. How does this clock work? In reality, each of our cells has one, through a system of genetic transcription and protein translation, whose cycle oscillates in 24 hours. Every one of these clocks is synchronised by a master clock located in the middle of the brain, in the suprachiasmatic nucleus." One of this clock's vectors is the regulation of a hormone, melatonin, secreted at night to make our body want to sleep. On the other hand, as soon as the body is exposed to light, the suprachiasmatic nucleus inhibits its secretion. A mechanism with a major influence on our sleep. "However, sleep researchers tend to forget this very old biological rhythm. Studies over the past few years have had a tendency to incriminate sleep debt to explain a whole series of types of cognitive deterioration."
During a study recently published in Science, a team of researchers from the University of Liège and the Surrey Sleep Research Centre at the University of Surrey in England, was able to highlight the effect of these two mechanisms on variations in cognitive performance.
42 hours without sleep
The effects of the circadian cycle are particularly complex to demonstrate experimentally. They are masked by other factors such as physical activity, diet, exposure to light, individual genetic profiles, etc. Each of these parameters influence our vigilance or the secretion of hormones such as adrenalin. "We wanted to study the variation in brain responses to simple tasks over a period of 42 hours without sleep", the neurologist continues, "to confirm that these changes were linked to sleep cycles. Therefore, we had to create a constant routine for the volunteers. This meant we had to put them in controlled conditions concerning light, temperature, diet, physical exercise, etc., so that the only variable during the experiment was the effects linked to sleep deprivation and the circadian clock."
The participants were all healthy and young. In the three weeks preceding the experiment, Pierre Maquet and his team recorded each of the potential participant's actigraphy. This actigraphy allowed them to check whether or not the subjects were respecting their sleeping times. "This measurement was essential to us. We can't assess the effect of sleep deprivation if we don't know the sleep history of the person studied. Knowing their circadian phase or ensuring that they aren't suffering from major sleep disorders are among the precautions required to be able to gather reliable data". Among the volunteers, 33 subjects were finally selected. After an adaptation night at the laboratory and one baseline night, the participants were kept awake for 42 hours, before a 12-hour recovery night.
The potential participants' sleep was closely monitored during the three weeks preceding the experiment. In the laboratory, the selected subjects had an adaptation night (A), a baseline night (B), 42 hours awake, and a 12-hour recovery night. During the 42 hours of wakefulness and after the recovery night, they were asked to do a series of tasks during 13 functional magnetic resonance imaging (fMRI) sessions in the CHU's cyclotron.
The subjects mainly stayed in their room for the whole 42 hours, and were constantly kept active to prevent them from falling asleep. During the experiment, they carried out repeated tasks 13 times during functional magnetic resonance imaging (fMRI) sessions. "While the participants were in their room", Pierre Maquet continues, "we repeated another series of tasks every hour, including a motor inhibition test. Successive figures appeared on the screen. According to the figure, the subjects had to press, or not press, on a button. Deprived of sleep, they experienced greater difficulty at inhibiting this movement, or on the contrary, their drowsiness prevented them pressing on the button for any of the figures. During the fMRI, they also took a series of tests that quantified their state of vigilance and their working memory. For instance, they were presented with a timer set at zero on the screen. As soon as it started running up the milliseconds, the subjects had to stop it as quickly as possible. During an auditory test, they heard a sequence of letters which they then had to tell us about. Was one letter the same as the one heard three items earlier? Etc." These sessions lasted about 10 minutes and required the subjects sustained attention. Maintaining such a level of vigilance is clearly difficult for someone who has been awake for more than 40 hours. But it is exactly these changes that help to quantify the effects of the circadian cycle and sleep deprivation.
The data collected was transferred to a timescale. The quality of the execution of the various tasks showed similar patterns that weren't in the least monotonic. "During the day, performance remained quite stable, but ultimately declined owing to sleep deprivation. We observed a sudden increase in reaction time after 22:00, a time that corresponds to the moment when the body starts to secrete melatonin."
The graph on the left illustrates the evolution in performance relating to the task measuring the reaction time. The grey curve corresponds to the melatonin secretion cycle. The hours are expressed in circadian terms (at the bottom of the x-axis), opposite the corresponding clock time (at the top of the x-axis). Time 0, corresponding to 22:00, indicates the beginning of melatonin secretion. The graph on the right illustrates subjective somnolence. The subjects had to assess their own vigilance on a scale of zero to nine. Zero referred to perfect vigilance and nine to the highest level of somnolence. The evolution of the two curves follows the secretion of melatonin, which falls slightly at the end of the first night to reach a stable level until dusk on the second day.
During the night, performance declines until early morning before improving, despite continued sleep deprivation. "Performance isn't as good as on the first day, which is normal", the researcher adds. "It's affected by sleep deprivation. However, it doesn't collapse in a linear manner. When the melatonin is inhibited again, the subjects recover part of their faculties for the day, before reaching a new and even bigger peak on the second evening. This data quite clearly shows an addition of incidences linked to the circadian rhythm on the one hand, and sleep deprivation on the other."
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."
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.