Circadian phenotype impacts the brain's resting state functional connectivity, attentional performance and sleepiness.

INTRODUCTION: Functional connectivity (FC) of the human brain's intrinsically connected networks underpins cognitive functioning and disruptions of FC are associated with sleep and neurological disorders. However, there is limited research on the impact of circadian phenotype and time of day on FC.

STUDY OBJECTIVES: The aim of this study was to investigate resting state FC of the default mode network (DMN) in Early and Late circadian phenotypes over a socially constrained day.

METHODS: 38 healthy individuals (14 male, 22.7 ± 4.2 years) categorised as Early (n =16) or Late (n = 22) using the Munich ChronoType Questionnaire took part. Following a two week baseline of actigraphy coupled with saliva samples for melatonin and cortisol rhythms, participants underwent testing at 14.00 h, 20.00 h and 08.00 h the following morning. Testing consisted of resting state functional MRI, a structural T1 scan, attentional cognitive performance tasks and self-reported daytime sleepiness. Seed based FC analysis from the medial prefrontal and posterior cingulate cortices of the DMN was performed, compared between groups and linked with behavioural data.

RESULTS: Fundamental differences in the DMN were observed between Early and Late circadian phenotypes. Resting state FC of the DMN predicted individual differences in attention and subjective ratings of sleepiness.

CONCLUSION: Differences in FC of the DMN may underlie the compromised attentional performance and increased sleepiness commonly associated with Late types when they conform to a societally constrained day that does not match their intrinsic circadian phenotype.