Abstract
Morning-type individuals experience more difficulties to maintain optimal attentional performance throughout a normal waking day than evening types. However, time-of-day modulations may differ across cognitive domains. Using functional magnetic resonance imaging (fMRI), we investigated how chronotype and time of day interact with working memory at different levels of cognitive load/complexity in a N-back paradigm (N0-, N2-, and N3-back levels). Extreme morning- and evening-type individuals underwent two fMRI sessions during N-back performance, one 1.5 h (morning) and one 10.5 h (evening) after wake-up time scheduled according to their habitual sleep–wake preference. At the behavioral level, increasing working memory load resulted in lower accuracy while chronotype and time of day only exerted a marginal impact on performance. Analyses of neuroimaging data disclosed an interaction between chronotype, time of day, and the modulation of cerebral activity by working memory load in the thalamus and in the middle frontal cortex. In the subjective evening hours, evening types exhibited higher thalamic activity than morning types at the highest working memory load condition only (N3-back). Conversely, morning-type individuals exhibited higher activity than evening-type participants in the middle frontal gyrus during the morning session in the N3-back condition. Our data emphasize interindividual differences in time-of-day preferences and underlying cerebral activity, which should be taken into account when investigating vigilance state effects in task-related brain activity. These results support the hypothesis that higher task complexity leads to a chronotype-dependent increase in thalamic and frontal brain activity, permitting stabilization of working memory performance across the day.
Highlights
Many factors contribute to daily decisions about when to go to bed and when to get up
A repeated-measures ANOVA was conducted on accuracy scores with within-subject factors working memory condition and time of day and between-subjects factor chronotype (Figure 1)
If considering performance on the N3-back condition separately, there was a significant interaction between chronotype and time of day [F(1,26) = 4.22, p = 0.05]
Summary
Many factors contribute to daily decisions about when to go to bed and when to get up. Extreme morning chronotypes are at one end of the continuum. They exhibit preference for waking up very early in the morning and find it difficult to remain awake beyond their usual bedtime [1]. At the opposite end of the continuum, extreme evening types strongly prefer to go to bed at late hours of the night and find it difficult to get up early in the morning. Accumulating evidence suggests that homeostatic sleep regulation differs between chronotypes. As quantified by electroencephalographic theta activity during wakefulness (6.25–9 Hz) and slow wave activity (SWA) during NREM sleep (1–5 Hz), homeostatic sleep pressure builds up [8] and dissipates [9,10,11] faster in morning than evening chronotypes
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