I used to be an owl but now I’m a lark!“Early to bed, early to rise, makes a man healthy, wealthy and wise”, according to the popular saying. Whether or not there is truth in this, a study carried out by the University of Liège’s (ULg) Centre de Recherches du Cyclotron today demonstrates that the way in which our brains work to focus our attention throughout the day changes according to whether we are “evening people” or “morning people”. It also shows that this difference is particularly marked at the end of the day when the pressure to sleep is higher, reflecting the number of hours spent awake.
This functional neuro-imaging study, published in the prestigious journal Science, is the result of a long-term project led by Christina Schmidt, FNRS research fellow at the ULg. The project was conducted under the supervision of Philippe Peigneux (Université Libre de Bruxelles) and Fabienne Collette (FNRS, ULg), in collaboration with Christian Cajochen (Basel University, Switzerland) and Pierre Maquet (FNRS, ULg).
Homeostatic vs circadian processes
Two processes operate to maintain the quality of our waking hours and of our cognitive performance during a normal day. On the one hand, our propensity to sleep progressively increases throughout the day and dissipates during the night – which is known as the homeostatic process. The intensity of the pressure to sleep which builds up during the day can be gauged by using electrodes to measure the density of slow waves characterising the electric activity in our brain during initial sleep cycles. In addition to the homeostatic process, our level of alertness is regulated by a circadian rhythm (i.e. +/- 24 hours), which increases our waking signal during the day and diminishes it once again at night. It is the balance between these two mechanisms (homeostatic and circadian) which enables us to retain a fairly stable level of performance in the various cognitive tasks we undertake in the course of a normal working day.
To better understand the cerebral bases of these regulatory mechanisms and the ways in which they interact, Christina Schmidt and her colleagues used the differences in rhythm between “morning” chronotypes and “evening” chronotypes. In fact, the “morning” subjects (or “larks”), who naturally go to bed early and wake up early, generally perform better in the morning, while the “evening” subjects (or “owls”), who find it more difficult to fall asleep until very late and who get up late when possible, feel in better shape in the evenings. The researchers recorded (or scanned) these chronotypes’ brain activity using functional Magnetic Resonance Imaging (fMRI) when performing a visual attention task. The task was carried out in two sessions, the first taking place 1.5 hours after waking and the second taking place 10.5 hours after waking, with subjects retaining their natural sleeping and waking pattern.
The results showed no difference in performance or brain activity between the two chronotypes in response to the attention-task after being awake for 1.5 hours, when the pressure to sleep is low. In contrast, under conditions where the pressure to sleep is heightened, i.e. after 10.5 waking hours, attention performance improves in evening chronotypes who outperform morning chronotypes. This increased alertness is linked to an increase in activity in the suprachiasmatic nucleus area (SCN) and the locus coeruleus (LC). These anatomically interconnected brain regions are heavily involved in the circadian signal which triggers waking and regulates the level of alertness upon waking.
These results suggest that brain responses may be due to differences in the way that morning and evening chronotypes react as pressure for sleep builds up throughout the day. In support of this hypothesis, Christina Schmidt’s results also demonstrate that despite the same number of hours spent awake, the “morning” chronotypes present denser slow sleep waves at the start of the night – in other words, they are more sensitive to homeostatic sleep pressure. What is more, this pressure to sleep is inversely related to the level of activity in the suprachiasmatic nucleus area during the alertness task. This demonstrates for the very first time in humans that the activity of the brain circuits responsible for circadian regulation is adjusted by the homeostatic processes of sleep. This suggests that “larks” (morning chronotypes) suffer more than “owls” (evening chronotypes) from the impact of the increased pressure to sleep as the day unfolds, pressure which prevents the optimal expression of the waking signal by the suprachiasmatic nucleus area and the locus coeruleus.
These new results demonstrate the direct influence of the complex interactions between homeostatic and circadian processes upon the brain activity which underlies human behaviour.
Source: Homeostatic Sleep Pressure and Responses to Sustained Attention in the Suprachiasmatic Area , Science, issue 24 April 2009.
