After-effects on cognition—where a prior activity either benefits or hinders subsequent cognitive performance—are empirically inconsistent. Do people have insight into when their subjective energy and cognition will be helped or hurt by engaging in prior activities? Studies 1a and 1b (combined N = 316) find that people expect more demanding and unenjoyable tasks to hinder their subsequent energy and cognitive performance, regardless of their willpower lay theory. Study 2 (N = 167) examines the accuracy of these forecasts using a within-subject design. Participants’ forecasts of their future subjective states did predict their actual experienced subjective states, but participants were not able to accurately forecast their subsequent math performance. Additionally, they significantly overestimated the detrimental effects of demanding prior activities on both subjective state and performance. Study 3 (N = 210) found that participants’ overestimation of detrimental after-effects could result in unnecessary financial costs, suggesting these biased forecasts can have consequences.