Primary insomnia (PI) sufferers typically complain of such daytime impairments as reduced attention, concentration, memory and global mental acuity. Moreover, epidemiological studies have shown PI contributes to reduced productivity, work and traffic accidents, and serious falls among the elderly. Despite such findings, laboratory-based efforts to corroborate the cognitive complaints of PI sufferers have produced mixed results. Indeed, many studies comparing PI sufferers with non-complaining normal sleepers across a range of neuropsychological tests have failed to show any relative deficits among the PI group. Such findings, in turn, has led to the impression that PI patients cognitive complaints may be over-stated and result from their attentional bias toward minor cognitive errors, dysfunctional beliefs about the impact of insomnia on functioning or excessive self focus rather than to any measurable daytime impairment.
However, many previous such studies were underpowered due to small sample sizes and employed neuropsychological tests designed for detecting impairment resulting from brain disease/damage rather than the more subtle albeit significant impairments of which PI patients complain. In recent research, we and others have shown that PI sufferers do, indeed, show greater deficits (slower and more variable reaction times) particularly on complex switching attention tasks. Moreover, there is some preliminary evidence that the subgroup of PI sufferers with elevated levels of physiological hyperarousal are most prone to suffer from neuro-cognitive performance deficits than are matched groups of PI sufferers who are not physiologically hyperaroused and normally alert individuals without insomnia. For example, Fernandez-Mendoza recently showed that PI sufferers with a hyperarousal pattern suggested by their objective short sleep duration on serial polysomnograms (PSG) performed more poorly on a complex switching attention task than did both normal sleepers and PI sufferers with normal objective sleep durations.
In our efforts to follow up on this latter work, we recently examined the error rates of alert and sleepy PI sufferers and normal sleepers across a series of simple and complex reaction time tasks. We employed age and gender matched samples of PI (N=89) sufferers and normal sleepers-NS (N=95). Participants underwent three nights of PSG followed by daytime testing with a four-trial Multiple Sleep Latency Test-MSLT. The PI and NS groups were each subdivided into "alert" (e.g., MSLT mean onset latency \> 8 minutes) and "sleepy" (e.g., MSLT mean onset latency \< 8 minutes) subgroups to allow for testing the main and interaction effects of participant type and level of alertness. "Alert" participants had longer MSLT latencies than "sleepy" participants (12.7 vs. 5.4 minutes). PI sufferers had fewer correct responses on performance testing than did NS. However, as shown by the adjacent, figure we found a significant group x alertness interaction (p = .0013) with greater error rates occurring among alert (hyperaroused) PI sufferers (Mean=4.5±3.6 errors per trial) than among alert NS (Mean=2.6±1.9 errors per trial). This was particularly true for the more complex switching attention task.
Our work along with that of Fernandez-Mendoza serve to confirm that PI sufferers have measureable objective neuro-cognitive deficits and provide some preliminary suggestion for the types of testing approaches that should be used to detect them. The identification of tests sensitive to PI sufferers' cognitive deficits are particularly relevant for testing the effects of current and future insomnia therapies on patients' objective daytime functioning. Measures of daytime dysfunction can and should serve as endpoints for assessing benefits and detriments of insomnia therapies. In addition, our recent work suggests that subgroups of PI sufferers may differ in their daytime deficits, with those showing physiological hyperarousal being most prone to make errors. This finding suggests that different types or doses of treatment may be needed to reverse the daytime impairments of the hyperaroused and non-aroused PI patients. However, our line of research would benefit by replication and extension findings to (1) further confirm the detrimental effects of physiological hyperarousal on PI sufferer's neuro-cognitive functioning; and (2) identify a broader range of tests that can be used for assessing diurnal cognitive impairments in both physiologically hyperaroused and lesser aroused PI groups. The current project will address these aims.
