Insomnia is a sleep disorder that is marked by difficulty in falling asleep, difficulty in maintaining sleep, and/or early morning awakening. Difficulty in falling asleep is particularly common in young adults, and sleep onset is affected by psychological factors. The purpose of the present study was to identify the physical and mental factors related to the subjective evaluation of falling asleep among Japanese university students. The participants were 366 students, including 197 (53.8%) females, with a mean age of 20.6 ± 1.7 years. The questionnaire battery mainly covered items about sleep onset, sleep quality, trait anxiety, and general mental state. Sleep onset was categorized as "easy to achieve" for 121 (33.1%) subjects, "difficult" for 38 (10.4%), and "intermediate" for 207 (56.6%). For example, "difficult" was defined as taking a longer time to fall asleep. The subjects with difficult sleep onset reported significantly higher awareness of the smell and noises in the bedroom, body sensations such as a heavy stomach feeling and frequent rolling over, mental agitation and excitement, unstable mental state, negative state, and strain. The subjects with difficult sleep onset also showed less sleep comfort and less recovery from fatigue. A multinominal logistic regression analysis revealed that each of body sensation, sleep comfort, unstable mental state, and fatigue influenced whether an individual had the difficult type. Anxiety-related factors at bedtime, in particular, may delay the sleep onset. The results of the present study indicate that many university students may be at risk of sleep-onset insomnia.