Second international School On Mind, Brain And Education

2007, May 22-26

Northwestern University. USA

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Time and teaching – the role of Chronobiology
Mental performance is influenced by many factors, including the time of day and the time that has elapsed since waking. It is these two factors that are the main subject of this presentation.
Effects due to time of day are mediated by the “body clock”. This clock is situated in the paired suprachiasmatic nuclei, found at the base of the hypothalamus. The cells within these nuclei show rhythmic patterns of “clock proteins”, in turn due to the rhythmic activation and suppression of “clock genes”. The cells’ output, measured as neurotransmitter release and firing rate, influences many variables through effects upon other regions of the brain. These variables include core temperature, hormones, food intake and the sleep-wake cycle. The rhythms of these variables produce widespread changes upon the body as a whole in, for example, mood and mental performance, and physical performance.
The properties of the body clock in humans cannot be measured directly, but must be inferred from measurements of “marker rhythms”. Important marker rhythms are those of core temperature, melatonin and the sleep-wake cycle. Investigation of such rhythms has indicated that the body clock normally runs slightly slower than the solar day (averaging 24.0 h) but shows an intrinsic period of about 24.3 h. However, the body clock is normally adjusted (entrained) to the solar day, this being achieved by rhythmic influences in the environment (zeitgebers) that feed, directly and indirectly, into the body clock.
Rhythmic changes in many types of physical performance and the abilities to fall asleep and to wake from it are timed similarly to the core temperature rhythm, and a causal link is generally assumed. For mood and mental performance, the position is rather more complex, factors in addition to the rhythm imparted by the body clock being present. This added complexity arises partly because of the wide range of tasks that is covered by the terms “mood” and “mental performance”, and partly by additional effects of time awake, time-on-task and sleep loss.
Mental performance tasks vary greatly in the amount of processing by the central nervous system (CNS) that is required, varying, for example, from simple reaction time to complex decision-making. There are also differences in the amounts of long- and short-term memory that are involved. In general, the more complex the task, the more susceptible it is to “exogenous” effects (of time awake, time-on-task and sleep loss) in addition to “endogenous” effects (of the body clock). Separating the contributions to a rhythm made by the endogenous and exogenous components requires a complex protocol, that of “forced desynchronisation”.
Other factors of chronobiological interest affect mental performance. These include: “sleep inertia” (the time it takes to “get going” after waking up), “the post-lunch dip” (a transient fall in mental performance in the early afternoon and made use of in those countries where a siesta is taken), and inter-individual differences (those between “larks”, morning-types, and “owls”, evening-types, being the best known).
Based upon such considerations, the chronobiologist is able to offer advice as to the best times for performing certain tasks and those times and circumstances when the tasks are likely to be performed less well.