Time-of-day variations of indicators of attention: performance, physiologic parameters, and self-assessment of sleepiness

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Background: A study was performed to analyze time-of-day variations of different indicators of attention and their interrelations. Methods: After a sufficiently long all-night sleep 12 healthy non–sleep-deprived subjects ran through a test battery (Stanford Sleepiness Scale, Visual Analogue Scale, Critical Flicker Fusion Test [CFF], Visualization Test, Number Facility Test, Reaction Time, Pupillometry, and modified Multiple Sleep Latency Test) every 2 hours from 7:00 am until 11:00 pm. Time-of-day variations were tested nonparametrically with Friedman’s test for repeated measurements. Principal component factor analysis (of individually standardized values) was used to identify variable complexes with the same pattern of time-of-day variation. Results: Statistically significant time-of-day variations were found for all variables, except for Fusion Frequency in CFF and Reaction Time. In factor analysis the physiologic parameters (pupillometric variables and sleep latencies) load on one factor, whereas the self-assessment scales, the Visualization Test, Number Facilty Test, and CFF load on the second factor. The variables that load primarily on factor 1 show peak levels of alertness immediately after getting up (at 7:00 am) and again at 9:00 pm. Those variables that load primarily on factor 2 indicate a peak level of alertness around noon (11:00 am–3:00 pm). Conclusions: Different aspects of attention follow different time-of-day variations. It is discussed, that these findings can be attributed to underlying circadian and homeostatic factors.