Many studies have established that insomnia is highly comorbid with psychiatric disorders and is a risk factor for the development of depression, anxiety, and suicide.83 However, the mechanisms by which insomnia precedes the development of psychiatric disorders, e.g., depression, are unknown. In a recent study from the Penn State Adult Cohort, insomnia with objective short sleep duration was associated with a psychological profile consistent with depressed mood, fatigue, concerns about health and physical functioning, somatically focused anxiety, and poor health status, which is typical of medical outpatients.58 In contrast, insomnia with normal sleep duration was associated with sleep misperception (i.e., the underestimation of time asleep and overestimation of time awake during the night) and a psychological profile consistent with depressed mood, rumination, anxiety, intrusive thoughts, and poor coping resources.58 These data have led us to suggest that both insomnia subtypes are associated with (or are at risk of developing) psychiatric disorders but that different pathophysiological mechanisms may account for such an association.59 For example, it is possible that biological mechanisms, i.e., hyperactivity of the HPA axis, may play a role in the development of depression in insomniacs with objective short sleep duration, while psychological mechanisms, i.e., poor coping resources and ruminative traits, may play such a role in insomniacs with normal sleep duration. However, these hypotheses need yet to be tested.