During mind wandering, attention is directed away from the external environment and cognitive processing is decoupled. Mind wandering is usually treated as a dichotomy and often measured using self-reports. We here propose the levels-of-inattention hypothesis, postulating graded decoupling at different processing levels. To measure levels of decoupling during reading we introduce the sustained attention to stimulus task (SAST), which relies on psychophysics of error detection. We found that subjects were less likely to notice errors at higher levels of cognitive processing. Eye tracking showed that before errors were overlooked effects of high- and low-level linguistic variables were reduced in a graded fashion, indicating episodes of weak and deep decoupling. Individual gaze durations predicted overlooking of errors five seconds before they occurred. Our findings support the levels-of-inattention hypothesis and suggest levels of mind wandering can be measured in the SAST. Eye tracking provides a promising tool to detect mind wandering online.