The journal Science has published an article entitled Homeostatic Sleep Pressure and Responses to Sustained Attention in the Suprachiasmatic Area, whose main author is Christina Schmidt, FNRS candidate at the ULg’s Cyclotron Research Centre. For the first time, functional magnetic resonance imaging has been used to study the influence of the chronotype on the way the brain functions in individuals faced with a cognitive task. The results: appreciable differences in terms of the level of activity in certain areas of the brain between ‘morning types’ and ‘evening types.’

All living beings possess an internal biological clock which allows them to adapt to periodical variations in their environment, in particular the changeover from day to night time. In humans, as in mammals, it comes in the form of an aggregate of neuronal cells located in the antero median section of the hypothalamus. Its name? The suprachiasmatic nucleus.

Its metabolic and electro-physiological activity has a natural rhythm over a period slightly greater than 24 hours. At least for the majority of us. This is what are called circadian rhythms: circa: around, diem: day). Nonetheless, in normal conditions elements of the physical and social environment named ‘time synchronisers’ recalibrate our biological clock to a scale of 24 hours. The principal one is constituted of variations in brightness caused by the changeover from day to night. But there are others, such as work schedules and meal times. Since its beginnings, chronobiology has studied numerous physiological variables from the perspective of circadian rhythms: body temperature, the production of cortisol, alertness, heart rate, mood, etc. It is also interested in cognitive performances, whose variable nature in terms of the time of the day has been well demonstrated.

A more subtle analysis nevertheless indicates that many factors need to be taken into consideration: first of all, the circadian rhythms; next, sleep pressure, or the need to sleep, linked to the number of hours the subject has remained awake. These two processes interact in concert or against each other according to the moment of the day to ensure an adequate awake-sleeping rhythm. The different inter-individual differences which modulate this interaction between circadian rhythm and sleep pressure also need to be taken into account. in fact there exist specific ‘chronotypes’ which can be inscribed on a Gaussian curve: 66% of the population is said to be neutral, in other words neither ‘morning’ or ‘evening’ types, whilst the remaining 34% are divided harmoniously into categories which we can term ‘moderate morning’, ‘moderate evening, ‘extreme morning’ (5%) and ‘extreme evening’ (5%) types.