European researchers have shed light on one of life's biggest mysteries: why do some people perform well in the wee hours of the morning, while others kick-start their energy in the evening? Led by Professor Christina Schmidt from the Cyclotron Research Centre at the University of Liège in Belgium, a team of researchers investigated how the length of time we have been awake and the time of day influence our alertness and ability to concentrate. The findings of the neuroimaging study have been published in the journal Science. 'Throughout the day, cognitive performance is under the combined influence of circadian processes and homeostatic sleep pressure. Some people perform best in the morning, whereas others are more alert in the evening,' the authors write in their paper. 'These chronotypes [i.e. whether you are a 'lark' or an 'owl' type] provide a unique way to study the effects of sleep-wake regulation on the cerebral mechanisms supporting cognition.' The researchers monitored the brain activity of extreme 'morning people' and 'evening people' by using functional Magnetic Resonance Imaging (fMRI). The subjects were instructed to live according to their own preferred sleep-wake schedule for at least one week before the study. During the study itself, the participants spent two consecutive nights in a sleep laboratory and were asked to carry out a task that called for sustained attention. They performed the task 1.5 hours after waking and again 10.5 hours after waking. No difference in performance or brain activity emerged between the two chronotypes in response to the task after the 1.5-hour mark. This was due to the fact that sleep pressure is low then. However, the researchers noted a change in performance or brain activity after 10.5 waking hours, due to greater sleep pressure. The circadian master clock is located in one of the brain activity areas. Consequently, the interactions between the brain regions controlling circadian rhythms and sleep pressure influence the behavioural differences between morning and evening people. The end result is that for the test taken 10.5 hours after waking, attention performance perked up for the evening people but not for the morning group. The circadian rhythm regulates the level of people's alertness on a 24-hour cycle. The waking signal intensifies during the day and weakens at night. At the same time, our propensity to sleep increases during the day and dwindles during the night; this is known as the homeostatic process. A balance between the circadian and homeostatic processes is what allows a person to maintain a steady performance level in the different cognitive tasks carried out within a 24-hour period, the researchers explained. Another interesting finding is how morning and evening people react as sleep pressure changes throughout the day. The researchers found that morning people showed denser sleep pressure at the start of night. In effect, they are more sensitive to homeostatic sleep pressure, which in turn is related to the level of activity during the alertness task in the part of the brain called the suprachiasmatic nucleus area. 'We found that maintaining attention in the evening was associated with higher activity in evening than morning chronotypes in a region of the locus coeruleus and in a suprachiasmatic area,' the authors wrote. 'Activity in the suprachiasmatic area decreased with increasing homeostatic sleep pressure. This result shows the direct influence of the homeostatic and circadian interaction on the neural activity underpinning human behaviour.' Also participating in the study were the Centre for Chronobiology at the Psychiatric Hospital of the University of Basel (Switzerland) and the Neuropsychology and Functional Neuroimaging Research Unit at the Université Libre de Bruxelles (Belgium).
Belgium, Switzerland, France