Background: Insomnia is common in patients with psychiatric disorders. However, patients' awareness of sleep has seldom been examined in detail. In this study, we investigated sleep awareness in outpatients at the psychiatry department of a university hospital. Methods: The participants (n = 241) were recruited at the psychiatry department of Ehime University Hospital between 11 October and 5 November 2021. The following questionnaires were used: Clinical Global Impression Scale of Severity (CGI-S), Global Assessment of Functioning (GAF), General Health Questionnaire (GHQ-30), Athens Insomnia Scale (AIS), and Epworth Sleepiness Scale (ESS). Psychiatric disorders were diagnosed by certified psychiatrists using the International Statistical Classification of Diseases and Related Health Problems 10. Participants with an AIS score of ≥6 were allocated to the insomnia group for statistical analysis. A logistic regression analysis was conducted to identify which items of sleep hygiene the patients with insomnia practiced using the Sleep Guidelines for Health Promotion. Results: Of 241 participants, 133 (55.2%) were allocated to the insomnia group. The mean scores for the CGI were significantly higher and the GAF scores were significantly lower in the insomnia group than in the healthy sleep group (p < 0.01). Of the 12 sleep guidelines proposed by the Japanese Government, "Do not go to bed until you are sleepful, do not delay getting up", was the item that maximally influenced insomnia. Conclusions: The insomnia group had worse scores on various medical assessment scales compared to the healthy sleep group. Based on a survey of outpatients at the psychiatry department of the university hospital, appropriate stimulus control techniques may help clinicians to treat outpatients with insomnia.