In the April, 2011 issue of Sleep a group of researchers examined the differences between individuals with a diagnosis of primary insomnia (PI) and a diagnosis of insomnia related to a mental disorder (IMD) on a number of self reported sleep measures and psychological tests. Subjects were identified as having a mental disorder based on the Structured Clinical Interview for DSM-IV, primarily depression, bipolar disorder, and anxiety disorders. All subjects completed a sleep history questionnaire, 2 weeks of sleep diary, polysomnograph, and several self report measures such as the Beck Anxiety Inventory and Beck Depression Scale.
Compared to IMD, PI exhibited lower night-to-night instability in total sleep time in their sleep diary. Ratings of pre-sleep emotional arousal also showed a lower instability across nights in the PI group. These differences in night-to-night variability suggested to these researchers that the 2 groups have different perpetuating mechanisms for their ongoing sleep difficulties. The increased variability of total sleep time in the IMD group could indicate the existence of a quicker and more efficient mechanism of sleep recovery in this group. Poor sleep on one night leading to a longer and more solid sleep the next night is thought to reflect the functioning of a sleep homeostat, with enhanced sleep debt and elevated sleepiness leading to improved sleep. This research supports to hypothesis that persons with primary insomnia have a dysfunctional sleep homeostat and may require more sleep debt to produce normal sleep. Treatment plans to improve the sleep of these 2 groups is likely to be different.