Over the past decades, there has been growing interest in whether the language people use (e.g., in social media) can reflect their well-being (WB) and ill-being (IB). However, little is known about how the content of spontaneous thoughts and experiences (e.g., reports of night-time dreams and daydreams) reflects WB and IB. The present thesis investigated whether emotional language use in dream and mind-wandering (MW; or daydreaming) reports reflects WB and IB. To this end, 1755 dream reports from 172 healthy adults and 1508 MW reports from 153 healthy adults were analyzed using the Linguistic Inquiry and Word Count (LIWC) text analysis software. Robust multiple linear regressions revealed that self-reported negative emotions over the past two weeks, as well as symptoms of anxiety and depression, predicted negative emotional language use in MW, but not dream, reports. Overall, anxiety accounted for the largest amount of variance in negative emotional language use. The findings carry theoretical implications regarding the nature as well as the function of spontaneous thoughts and experiences, while also paving the way for new diagnostic and prognostic tools in clinical contexts. Additional research is required to confirm the observed relationships and establish their causal direction.