Time of day and chronotype in the assessment of cognitive functions.

Purpose: Circadian rhythms are synchronized, through the endogenous biological clock, with the 24-hour cycle, and associated with numerous changes in human functioning, both in physical and mental aspects. It is assumed that daily fluctuations in cognitive performance are a consequence of interacting homeostatic and circadian processes regulating an individual's sleep-wake rhythms. A chronotype may be understood as a phenotype of circadian cycles determined by an endogenous biological clock. Despite research findings showing a significant relationship between those factors and cognition, they remain insufficiently considered in the domain of cognitive psychology and neuropsychology. Views: This narrative review aims to describe and highlight the interactions between the time of day, chronotype, and cognitive performance in domains of mental activity variables, receptive functions, memory and learning, expressive functions, and thinking. Conclusions: The results of the research show that time-of-day effects on basic and more complex cognitive functions depend on an individual's chronotype with a dominant effect of synchrony between chronotype and time of cognitive testing, with large differences in circadian cycles between younger and older age groups. It is suggested that chronotype assessment and time of day control should be included as important variables in the diagnosis of cognitive function in both healthy and clinical populations. There is also an emerging need for further investigations to better understand how chronotype and circadian rhythms modulate human brain physiology and cognition.