Symptoms may include difficulty falling asleep, trouble staying asleep, and not feeling restored by sleep, although it has not been clear that it is possible to identify distinct subtypes of patients by symptom or that distinguishing symptom type affects the course of clinical treatment. Classification of insomnia by duration most commonly involves three categories: transient (no more than a few days), short-term (up to 3 weeks), and long-term (more than 3 weeks). This categorization is of uncertain utility and has been primarily based on nonempiric concerns about treatment with sedative-hypnotic medications for periods longer than several weeks. The subtyping of insomnia in terms of whether there is an identifiable underlying cause such as a psychiatric or medical illness was based on an unproven assumption that in most instances other disorders caused insomnia.