The results of the experiment? There was no observed difference in the levels of performance or cerebral activity between the two categories of subject (‘extreme morning’ and ‘extreme evening’ types) when the test was carried out 1h30 after waking, in other words at a moment when sleep pressure is low. On the other hand, 10h30 after waking, under greater sleep pressure, the attention giving performance improves amongst subjects who have an ‘extreme evening’ chronotype, which is not the same case for their ‘extreme morning’ counterparts. At the same time the suprachiasmatic nucleus (SCA – circadian biological clock) and the locus coeruleus (LC), two regions of the brain anatomically inter-connected and heavily involved in the circadian signal which promotes wakefulness and regulates our vigilance during the wakeful state, experienced an increase in activity amongst the evening subjects.

‘On the basis of this data, our hypothesis was that sleep pressure accumulates in a quicker manner for ‘the morning subjects’, in such a way that they are more tired than ‘the evening subjects’ when they have spent the same number of hours in a waking state,’ points out Christina Schmidt. There exists an excellent indicator of sleep pressure: the density of slow waves (recorded by electroencephalography) during the first sleep cycle. And, as a result, it appeared that the subjects equipped with a morning chronotype where characterised by a strong density of such waves at the beginning of the night.

From which the researchers’ conclusion: ‘Sleep pressure is inversely linked to the level of activity in the region of the suprachiasmatic nucleus during the vigilance task, showing for the very first time in humans that the activity of brain circuits responsible for circadian regulation is modulated by the homeostatic processes of sleep. This suggests that ‘morning subjects’ suffer more strongly than ‘those of the evening’ from the sleep pressure accumulated over the course of the day, a pressure which prevents the optimal expression of the wake promoting signal in the regions of the suprachiasmatic nucleus and the locus coeruleus.’

More complex tasks

The work of the Belgo-Swiss team have not stopped when presented with such a good pathway. Beyond the (simple) visual attention task, the researchers offered the two groups of 16 selected volunteers a task involving short term memory and another called conflict management, bringing into play the executive functions.

For the short term memory task the participants were placed in three different situations. In the first they had to indicate, using two buttons, one for ‘yes’ and one for ‘no’, if a letter that was presented to them on a screen corresponded to a predefined letter (T, for example). This was a control condition allowing the brain areas which were activated on seeing the letters to be determined, without a short term memory task being prescribed. In the second situation, where letters were made to appear successively, the participants were asked to decide if the letter they were seeing corresponded or not to the letter which was fixed on the screen two ‘times’ beforehand. Thus a mnestic load was added to the control condition. A load which was increased in the third situation, in which it was necessary to refer to the letter which appeared three ‘times’ beforehand. Here as well the tests were carried out, for each participant, 1h30 and 10h30 after their usual waking up time. The results (2), which have not yet been analysed in detail nor published, seem to corroborate the results received for the preceding test (visual attention). Nonetheless the differences between ‘morning subjects’ and ‘evening subjects’ no longer focuses on the reaction times but on the number of correct responses given.

(2) The results of the fMRI are in particular are in the process if being analysed.