Arousal and sustained attention fluctuate differently with respiration in younger and older adults.

Respiration is being increasingly recognised as both synchronising its dynamics with external events and modulating internal psychophysiological states. However, the extent to which these effects stem from a respiratory modulation of attention remains underexplored. Here, we leverage differing attentional strategies of younger (YA) and older adults (OA)-OA exhibited greater focus during a simple contrast change detection task-to examine their relationship with respiratory phase-locking behaviour. OA exhibited stronger phase-locking of their respiratory cycle to task-relevant events compared with YA. Notably, participants appeared to actively adjust their breathing so that late exhalation phases coincided with target presentation, despite variable inter-target intervals. To characterise this target-locked respiratory phase window, we analysed pupil diameter and EEG frequency-power as indices of arousal and attention. Pupil diameter, frontal delta and theta, posterior alpha, and steady-state visually evoked potential (SSVEP) amplitude all varied significantly over the respiratory cycle, suggesting that arousal was enhanced for respiratory phases aligned with target expectancy and attenuated outside these phases. OA showed stronger respiratory modulation of delta, theta, and alpha, whereas YA showed stronger modulation of pupil diameter and SSVEP. We interpret these findings as evidence that respiration shapes attentional fluctuations, expanding and contracting the vigilant state across the respiratory cycle through interactions with arousal and attentional systems. Further, the age-dependent quality of attention which is applied to a task has implications for the degree of respiratory phase-locking and how physiological signatures of arousal and attention are modulated.