Abstract
ObjectiveBased on individual daily physiological cycles, humans can be classified as early (EC), late (LC) and intermediate (IC) chronotypes. Recent studies have verified that chronotype-specificity relates to performance on cognitive tasks: participants perform more efficiently when tested in the chronotype-specific optimal time of day than when tested in their non-optimal time. Surprisingly, imaging studies focussing on the underlying neural mechanisms of potential chronotype-specificities are sparse. Moreover, chronotype-specific alterations of language-related semantic processing have been neglected so far.Methods16 male, healthy ECs, 16 ICs and 16 LCs participated in a fast event-related functional Magnetic Resonance Imaging (fMRI) paradigm probing semantic priming. Subjects read two subsequently presented words (prime, target) and were requested to determine whether the target word was an existing word or a non-word. Subjects were tested during their individual evening hours when homeostatic sleep pressure and circadian alertness levels are high to ensure equal entrainment.ResultsChronotype-specificity is associated with task-performance and brain activation. First, ECs exhibited slower reaction times than LCs. Second, ECs showed attenuated BOLD responses in several language-related brain areas, e.g. in the left postcentral gyrus, left and right precentral gyrus and in the right superior frontal gyrus. Additionally, increased BOLD responses were revealed for LCs as compared to ICs in task-related areas, e.g. in the right inferior parietal lobule and in the right postcentral gyrus.ConclusionsThese findings reveal that even basic language processes are associated with chronotype-specific neuronal mechanisms. Consequently, results might change the way we schedule patient evaluations and/or healthy subjects in e.g. experimental research and adding “chronotype” as a statistical covariate.
Highlights
Chronotype-specificity is characterised by individual preferences in sleep and wakefulness that reflect endogenous, self-sustained genetic dispositions [1]
There are no patents, products in development or marketed products to declare. This does not alter the authors' adherence to PLOS ONE policies on sharing data and materials. These findings reveal that even basic language processes are associated with chronotypespecific neuronal mechanisms
Chronotypes are grouped as intermediate chronotypes (IC) that have the highest prevalence in the population [2], late chronotypes (LC) that go to bed late at night and have difficulties getting up in the morning, and early chronotypes (EC) that tend to wake up at very early hours and find it difficult to remain awake beyond their usual bedtime
Summary
Chronotype-specificity is characterised by individual preferences in sleep and wakefulness that reflect endogenous, self-sustained genetic dispositions [1]. Previous studies have determined a time window of approximately three hours during subjective evenings which is characterised by maximal circadian wake promotion, i.e. the existence of a powerful drive for wakefulness between ten to twelve hours after awakening [9, 10]. During this time window, the so-called “wake maintenance zone” [9], humans are assumed to be prevented from falling asleep early in the evening hours– the homeostatic sleep pressure is at its highest level
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