Background <p>To investigate the regulatory role of sleep in the effect of preoperative intrusive thoughts on the fear of pain.</p> Methods <p>A descriptive, cross-sectional study and complying with STROBE guidelines. The study included 209 patients hospitalized in a tertiary hospital during the preoperative period. Data were collected using a Patient Identification Information Form, the Fear of Pain Questionnaire-III, The Preoperative Intrusive Thoughts Inventory (PITI), and the Richards-Campbell Sleep Questionnaire (RCSQ).</p> Results <p>Female gender and the severity of pain preoperatively were determined to have an effect on the fear of pain and preoperative intrusive thoughts (<i>p</i> &lt; 0.05). The fear of pain was determined to be negatively correlated with sleep quality (<i>r</i>=-0.172, <i>p</i> &lt; 0.05), and positively correlated with preoperative intrusive thoughts (<i>r</i> = 0.579, <i>p</i> &lt; 0.001). Sleep quality (<i>β</i>=-0.165, <i>p</i> &lt; 0.001) and preoperative intrusive thoughts (<i>β</i> = 0.850, <i>p</i> &lt; 0.05) were determined to be predictive of the fear of pain. There was not observed to be any regulatory role of sleep quality in the effect of preoperative intrusive thoughts on the fear of pain (<i>β</i> = 0.001, <i>p</i> &gt; 0.05).</p> Conclusion <p>The preoperative intrusive thoughts of patients and their sleep quality affect the fear of pain. Identifying and controlling the factors affecting the fear of pain will reduce the postoperative pain levels of patients, prevent complications.</p>