Are you an evening or a morning person?
At the University of Liège, Christina Schmidt took a different path. In effect, the work which enabled her to figure as the major author of the article entitled Homeostatic Sleep Pressure and Responses to Sustained Attention in the Suprachiasmatic Area(1), published on the 24th of April 2009 in the journal Science, put on the stage more ‘marginal’ subjects, in other words people who are ‘extreme morning types’ or ‘extreme evening types.’ In addition they were young individuals (18-30 years old). Why? Because beyond the age of 30 the parameters of sleep tend to change – with age an ever increasing propensity to ‘become morning types’ manifests itself. Two groups of 16 volunteers were established, one made up of ‘extreme morning types’, the other of ‘extreme evening types.’ ‘The selection was carried out on the basis of a questionnaire to which 6000 people responded,’ reports Christina Schmidt. ‘Different variables were taken into consideration, because we need to be careful of misleading appearances. Thus, certain people get up early because they are obliged to due to their work schedule. The people we were interested in for the ‘extreme morning’ were not these people, but those who rise early spontaneously, without an alarm clock, even at the weekend. In the same way, the ‘extreme evening types’ are people who, when they have the opportunity, watch television until three or four o’ clock in the morning and do not leave their beds before around 11 o’ clock in the morning.’
Another special feature of the work reported in the 24th of April issue of Science is that it calls on functional magnetic resonance imaging (fMRI), whereas all the studies previously carried out on the relationships between biological rhythms and cognition were based on behavioural issues.
The influence of the chronotype
At the heart of a collaboration between the University of Liège (Cyclotron Research Centre, department of Cognitive Sciences) and the University of Basel (Centre for Chronobiology), the programme managed by Christina Schmidt under the supervision of Professor Philippe Peigneux (ULB) and Fabienne Collette (ULg), with the help of Pierre Maquet (ULg) et de Christian Cajochen, has thus delivered its first results. They relate to a visual attention task bringing into play the subject’s reaction times. What did it consist of? The volunteers had to focus on a cross on a computer screen. From time to time, and in random fashion, the cross disappeared and gave way to a digital countdown. It was thus a question of pushing a button as quickly as possible in order to stop it. At the same time the subject’s brain activity was recorded by fMRI. The test, which was carried out twice over the course of a day, lasted 10 minutes and numbered 90 events (appearances of the countdown). They did not occur at just any old time. No, the challenges were offered to each volunteer one and a half and ten and a half hours after the time of day they usually wake up at.
This original procedure offers two essential advantages. On the one hand it enables an equalling out between the subjects of the number of hours spent in a state of wakefulness and the sleep inertia which follows waking up, two variables which can influence the results. On the other hand, it takes into account the fact that the evening and morning subjects could be differentiated by their state of vigilance at specific times of the day. ‘Without controlling this aspect, we could not exclude the hypothesis that the chronotypical variability of the performances concerning the prescribed task would only be secondary to a variation in the levels of vigilance,’ explains Christina Schmidt.
(1) Schmidt, C., Collette, F., Leclercq, Y., Sterpenich, V., Vandewalle, G., Berthomier, P., Berthomier, C., Philipps, C., Tinguely, G., Darsaud, A., Gais, S., Schabus, M., Desseilles, M., DangVu, T., Salmon, E., Balteau, E., Degueldre, C., Luxen, A., Maquet, P., Cajochen, C., & Peigneux, P. (2009). Homeostatic Sleep Pressure and Responses to Sustained Attention in the Suprachiasmatic Area. Science 324, 516.