Vulnerability in Vigilance Fluctuations During Daytime: Evidence From Behaviour and Resting‐State <scp>EEG</scp>

ABSTRACT While the significance of adequate nocturnal sleep for daytime vigilance is well recognised, substantial inter‐individual variation exists in vigilance fluctuations across the day. In a laboratory‐controlled setting, we examined individual differences in daytime vigilance fluctuations and dissociated the contributions of sleep pressure and circadian rhythm. With 48 healthy adults, we recorded resting‐state electroencephalogram (EEG) activity and subsequent performance on the Psychomotor Vigilance Task (PVT) in the morning (MOR), the afternoon without napping (NN) and the afternoon after napping (NP). Based on the changes in PVT performance between MOR and NN, participants were divided into a vulnerable group (24 individuals) and a resilient group (24 individuals). From MOR to NN, vulnerable individuals exhibited broader increases in delta, theta, beta1 and beta2 power across distributed regions under elevated sleep pressure. These broad spectral changes contributed to a progressive decline in vigilance, which can be reversed by a post‐lunch nap to dissipate accumulated sleep pressure. In contrast, resilient individuals maintained stable vigilance by selectively enhancing frontal theta or beta activity. These distinct neural characteristics may serve as biomarkers for predicting fatigue vulnerability and guiding personalised countermeasures in safety‐critical occupations.