Neuroscience is beginning to uncover the role of interoceptive feedback in perception, learning, and decision-making; however, the relation between spontaneous visceral and cognitive dynamics has received surprisingly little scrutiny. Here, we investigate how subjective, physiological, and behavioural indicators of arousal and attentional state vary as a function of cardiac activity and brain-heart coupling. Combined electroencephalogram, electrocardiogram, and pupillometric records were obtained from 25 healthy human adults during the performance of a sustained attention to response task (SART). Thought-probes were intermittently administered during the SART to collect subjective reports of attentional state (i.e., on-task, mind-wandering, mind-blanking) and vigilance level (i.e., alertness). Mind-wandering and mind-blanking reports increased in frequency with time-on-task, but were associated with distinct behavioural profiles. Mind-blanking was further characterised by more profound decreases in heart-rate and pupil size than mind-wandering, and late modulation of the heartbeat-evoked potential. Besides attentional state, lower heart-rate predicted decreased vigilance and pupil size, while heart-rate variability predicted more impulsive behaviour and pupil dilation. Together, these findings reveal that cardiac parameters afford complementary information about arousal states and attentional dynamics, illustrating the complexity of task disengagement from a cognitive and physiological perspective.