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. 

(1) Vincenzo Muto, Mathieu Jaspar, Christelle Meyer, Caroline Kussé, Sarah L. Chellappa, Christian Degueldre, Evelyne Balteau, Anahita Shaffii-Le Bourdiec, André Luxen, Benita Middleton, Simon N. Archer, Christophe Phillips, Fabienne Collette, Gilles VandewalleDerk-Jan Dijk, Pierre Maquet,Local modulation of human brain responses by circadian rhythmicity and sleep debt, Science, Vol. 353, Issue 6300, pp. 687-690, 12 Aug 2016