Etiology and pathogenesis
The etiology of paradoxical insomnia is not known, and the neurophysiological mechanisms of paradoxical insomnia still require further investigation (32). Some attribute the insomnia complaint to presleep cognition that imparts an impression of not sleeping. Others theorize that excessive mentation in sleep gives the patient the impression of not sleeping although, in fact, the mentation is in the sleep state. A report found that insomnia patients who were unable to identify the cause of their sleep disturbance were most likely to underestimate their sleep (12). Paradoxical insomnia and psychophysiological factors have overlapping symptoms as well as similarly elevated adrenocorticotropic hormone and cortisol levels as measured in a small study comparing the 2 groups (25).
In a similar vein, the disorder is seen as an extreme example of the tendency of most insomnia patients to exaggerate or misreport their sleep problem. Researchers have reported that patients with primary insomnia have attenuation in the mesograde amnesia that accompanies sleep and are more likely to recall information from sleep onset intervals than control subjects (30). It has been hypothesized that psychological distress influences perception of sleep, which is supported by the finding that elevated scores on the Psychasthenia scale of the Minnesota Multiphasic Personality Inventory correlate positively with sleep time underestimation. Others speculate that a physiological abnormality exists in these patients, but that it is too subtle to be detected by conventional polysomnography (18; 24). For instance, it has been shown that auditory stimuli sufficient to increase arterial blood pressure or heart rate without signs of arousal on the EEG can result in increased sleepiness. Actigraphy has shown increased movement in paradoxical insomnia patients, providing some support for this hypothesis.
In some patients with subjective sleepiness, misperception or mislabeling of another bodily state such as fatigue or depression probably accounts for the symptom. For instance, in 2 studies, both nefazodone and fluoxetine alleviated depression and resulted in improvements in subjective sleep quality, although patients given fluoxetine showed significant declines in objective sleep characteristics such as an increased number of awakenings and decreased sleep efficiency. No group difference in sleep diary measures of sleep quality was seen between patients given nefazodone or the antidepressant paroxetine, despite the declines in objective sleep measures observed in patients given paroxetine. Others may attribute such daytime dysfunction as trouble concentrating or memory problems to sleepiness when in fact sleepiness is not present. Alternatively, the usual changes in alertness that are related to the circadian rhythm can be perceived as abnormal somnolence by some patients when those are expected changes in the levels of sleepiness.
Patients with posttraumatic stress disorder underestimate total sleep time and sleep efficiency compared to objective sleep measured by actigraphy (15).
Sleep apnea patients are found to overestimate sleep-onset latency and underestimate total sleep time (10). In a large sample of patients with obstructive sleep apnea (n=405), perception of total sleep time improved when the patient underwent a CPAP titration study (10). These reports support the concept of paradoxical insomnia in somnolent patients. In the above cited reports, the authors conclude that marked discrepancies between patient reports and objective measures are not limited to insomnia patients. The subjective state of sleepiness is correlated with, but different than, physiological sleepiness or sleep propensity as measured by the multiple sleep latency test.
Patients with mild traumatic brain injury have been shown to estimate that they sleep a bit less than they actually do on PSG than perceived. A sample of 37 patients reported they slept 342 minutes in the lab when they actually slept 382 minutes on average (35).
The essential aspects of paradoxical insomnia are that physiological measurement of the sleep-wake system is intact, despite the complaint of sleep-related symptoms. The pathogenesis and pathophysiology for the perception of sleep and sleepiness remains unclear. An attempt has been made to correlate multiple EEG measures such as sleep stage, sleep onset latency, waking time and arousal frequency, total sleep time, EEG frequency, and sleep spindle density with the perception of sleep. It is clear that no single factor or combination of EEG factors completely explains the subjective perception of sleep. Likewise, the pathogenesis and pathophysiology for the misperception is unknown. However, some progress has been made in terms of the role of sleep spindles in paradoxical insomnia. Normand and colleagues suggest that a deficiency in sleep spindles might account, at least in part, for the sleep state misperception that occurs in paradoxical insomnia (27). Sleep spindle characteristics for 17 patients with paradoxical insomnia, 29 good sleepers, and 24 patients with psychophysiological insomnia were compared. Relative to the other groups, paradoxical insomniacs had the shortest sleep spindles on night 3 of 4 in the laboratory. The study highlights a need for additional studies on spindle length and its role in perception of sleep.
Perception of being asleep or awake in insomnia patients differs from the perception in normal sleepers (23; 22), and this is independent of age, sex, or laboratory versus ambulatory recording setting (34). In Trimmel and colleagues’ study, n = 303 patients with insomnia underestimated total sleep in the sleep laboratory by an average of 46 minutes. Paradoxical insomnia was not specifically studied in these reports, but these findings may point to a unique perceptual difference in insomnia patients. One paper investigated insomnia patients with paradoxical insomnia and reported differences in a physiological measure of arousal on comparing these patients to normal sleepers and objectively documented primary insomniacs (07). Another study compared REM sleep macrostructure and microstructure as well as clinical interviews between good sleepers, psychophysiological insomnia, and paradoxical insomnia. Participants underwent 4 consecutive PSGs in the laboratory. The paradoxical insomnia group was distinguished by higher number of wake intrusions in REM, and there was a relationship between REM duration and dream imagery activity (31).
Insomnia patients with less alpha EEG activity during recorded sleep have been reported to underestimate sleep time and overestimate intermittent awakening when compared to a group of insomnia patients with increased alpha activity (33). The aforementioned findings support an underlying neurophysiological mechanism in paradoxical insomnia.
Alpha-delta sleep (A-DS), which is an electroencephalographic (EEG) feature characterized by presence of alpha waves and low voltage fast activity representing relaxed wakefulness, superimposed on delta waves, characteristic of deep nonrapid eye movement (NREM) sleep, has long been associated with light sleep sensation. The literature regarding A-DS and sleep time misperception has been equivocal. However, in 1 study, investigators prospectively analyzed polysomnograms of about 5000 patients referred to a sleep laboratory and showed significant correlation between the discrepancy in subjective and objective sleep duration and perception of light sleep with degree of A-DS ranked from 1 to 4, especially in patients without obstructive sleep apnea and insomnia (21).
EEG spectral analyses of PSG recordings in insomnia patients have been explored in a number of studies. Some of the investigations showed greater relative amplitude of the high frequency (14 to 45 Hz) range as well as excessive high frequency activity amongst the subjective insomnia patients, suggesting a relationship to their sleep complaint (29; 20).
Cyclic alternating pattern (CAP) is an EEG pattern characterized by sequences of electrocortical events of non-REM sleep distinct from background activity and recurring with a periodicity of 20 to 40 seconds. This has been reported to correlate with poor sleep quality. Patients with paradoxical insomnia had significantly higher CAP rate overall, which increased before subjective sleep onset time compared to normal controls (28). These findings were supported in a study that compared CAPs in 10 individuals with paradoxical insomnia to 10 individuals with psychophysiological insomnia and to 10 normal controls. Differences mostly concerned increases in A3 and B3, and especially B3 in stage 2, in both psychophysiological insomnia and paradoxical insomnia. The authors attributed these findings to the level of hyperarousal. Moreover, although CAP rate was similar between the insomnia subtypes, the overall CAP time was increased in the paradoxical insomnia subgroup (11).
It is important to keep in mind that even in normal healthy volunteers, misperceptions of sleep can occur. In a sample of 44 participants, all were asked to sleep at their normal nighttime schedule on their first night in the laboratory. Then they had a 12-hour nighttime sleep opportunity as well as a 4-hour daytime nap opportunity without access to time cues. The extended night sleep opportunities and long daytime naps were designed to induce an “insomnia” with substantial time awake during scheduled sleep. Results showed that sleep estimations were accurate when participants sleep according to their normal night’s schedule. Participants underestimated sleep time in the 12-hour sleep opportunity, and this was inversely correlated with stage 2 sleep. Participants overestimated sleep in the 4-hour daytime nap opportunity, and this was correlated with REM and stage 3 sleep. The study suggests that sleep misperceptions may be related in part to the presence or absence of time cues and time of day (06).
Further studies are needed for better understanding of the underlying pathophysiology of paradoxical insomnia, which should include, but not be limited to, quantitative EEG analysis, neurophysiological, and objective psychological variables. Evaluating the impact of various stimuli on cortical measures such as examining event-related potentials (ERPs) to study hyperarousal is also recommended (05). By the same token, more patients evaluated for subjective complaints of excessive sleepiness or perceived insomnia should be evaluated with polysomnogram studies and multiple sleep latency tests, in order to correctly identify paradoxical insomnia.