The performance of the human visual system exhibits moment-to-moment fluctuations influenced by multiple neurocognitive factors. To deal with this instability of the visual system, introspective awareness of current visual performance (metacognitive monitoring) may be crucial. In this study, we investigate whether and how people can monitor their own visual performance during sustained attention by adopting confidence judgments as indicators of metacognitive monitoring - assuming that if participants can monitor visual performance, confidence judgments will accurately track performance fluctuations. In two experiments (N