Abstract :
[en] Although it has been established that human brain physiology and cognition are under the joint effect of the sleep homeostasis and the circadian alerting signal, the detrimental effect of sleep deprivation is still mostly seen as merely a consequence of a lack of sleep. While this approach is valuable, in order to develop a complete understanding, a circadian perspective needs to be integrated. However, a major difficulty of measuring circadian rhythmicity stems from the complexity of assessing it, because confounders such as light, activity, meals etc. could mask the underlying circadian regulation. Here, we performed two constant routine studies that allow us to measure the interaction between sleep homeostasis and the circadian processes at the cortical level. During the studies, three complementary aspects of the cortical function were investigated, as well as their associations with behavioural performance, and age-related changes of the cortical dynamics. In phase I of the study, the dynamics of cortical excitability, and of response scattering and complexity were described during a 28 hour wake extension protocol in young participants (18-30 y). In phase II, the dynamics of cortical excitability and response complexity were investigated during a 34 hour wake extension in young (18-30 y) and older (50-70 y) participants in order to address lifetime changes. Overall, the results of this thesis demonstrated an age-dependent homeostatic and circadian regulation of basic cortical function. That was especially evident at the local level, when focusing on cortical excitability profile: young participants showed a clear circadian rhythmicity and sleep homeostasis regulation, the dynamic of which was dampened in the older participants. At the global level, cortical response scattering and complexity changed with time spent awake, i.e. according to the circadian phase Furthermore, cortical complexity response was higher in the older group, showing a simple age effect, but the dynamic did not differ between the two age groups. Preliminary analyses demonstrated that these cortical dynamics sustain part of the profile of behavioural performance across the circadian cycle. Importantly, older people with higher cortical excitability, particularly during the biological night, were performing better at higher order tasks, possibly indicating that older people that maintain a degree of sensitivity toward sleep homeostasis and circadian processes perform better. Understanding the principal forces that regulate the dynamics of cortical neurophysiology in two age groups –and their impact on cognition– is of uppermost importance for our ageing society, in which sleep deprivation and circadian misalignment are commonplace.
