To further the understanding of illusory correlations, relationships between circadian variations (morningness-eveningness), personal need for structure, and the development of illusory correlations were examined. Consistent with predictions, U.S. university students who participated at nonoptimal times (morning types at night; evening types during the day) formed significant illusory correlations based on a percentage estimate measure; participants during optimal periods did not. Significant illusory correlations were found also among participants with a high need for personal structure but not among participants with a lower need for such structure. Comparisons based on trait ratings and a cued recall (assignment) task were not significant. Overall, the findings are consistent with the idea that illusory correlations are by-products of a cognitively economical heuristic process.