Etiology and pathogenesis
The etiology of paradoxical insomnia is not known, and the neurophysiological mechanisms of paradoxical insomnia still require further investigation (28). Some attribute the insomnia complaint to presleep cognition that imparts an impression of not sleeping. Others theorize that excessive mentation or rumination in sleep gives the patient the impression of not sleeping although, in fact, the mentation is in the sleep state. Sleep state discrepancy has been associated with certain personality traits, such as neuroticism and hypochondriasis, which can increase feelings of anxiety and arousal. This has typically been seen in paradoxical insomnia where polysomnography findings are normal (31). 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 two groups (22).
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. Insomnia patients, including cases of subjective insomnia, tend to underestimate their total sleep duration (32). Individuals with insomnia can be more prone to sleep fragmentation and may need longer periods of continuous sleep (greater than 30 minutes) to perceive they were asleep (07).
Investigators have hypothesized that there are differences in brain anatomy and function in patients with paradoxical insomnia compared to healthy subjects. Researchers have reported that patients with paradoxical insomnia were found to have regional shrinkage in specific areas of the brain, including the caudate, nucleus accumbens, putamen, hippocampus, thalamus, and amygdala (13). The direction of causality of these findings is unclear; their relationship to the estimation of sleep has yet to be explained.
Psychological distress from certain personality traits may influence 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 there are certain neurophysiological mechanisms of sleep misperception in these individuals, including abnormal neuronal circuitry and increased cortical arousals (29). 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 addition, certain personality profiles have been linked to paradoxical insomnia, such as anxiety, rumination, intrusive thoughts, and depressed mood. Pathological traits, including neuroticism, excessive body dissatisfaction, or histrionic somatization, have been seen in insomnia patients. Patients with paradoxical insomnia display anxiety and subjective uneasiness about their sleep, which could eventually lead to sleep difficulties viewed objectively. Therefore, the results of the Minnesota Multiphasic Personality Inventory may actually differ if administered when symptoms of insomnia first appear compared to later on (29).
Emerging research and neurophysiologic studies have shown altered EEG frequency bands and event-related potentials in nonrapid eye movement sleep in patients with paradoxical insomnia (31). This could be indirectly linked to increased microarousals in these patients, which could contribute to subjective feelings of sleepiness and decreased total sleep time.
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 two 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.
Studies now show that sleep-wake estimation inaccuracy is a common feature throughout sleep-wake diagnoses (32).
Patients with sleep apnea are found to overestimate sleep onset latency and underestimate total sleep time (11). 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 (11). 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.
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 (24). 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.
A study by Senel and colleagues looked at the role of sleep spindles in paradoxical insomnia (06). The study looked at sleep spindle characteristics in 20 patients with paradoxical insomnia versus 20 healthy control subjects. Additionally, certain personality traits were assessed via several questionnaires, including neuroticism or extroversion (Eysenck Personality Questionnaire), somatic perceptions (Somatization Scale), uncontrollable anxiety (Penn State Worry Questionnaire), tendency for ruminative thinking (Ruminative Thought Style Questionnaire), and depressive symptoms (Beck Depression Inventory). A significant difference was observed in the density of sleep spindles between the two groups of patients, with lower density spindles observed in patients with paradoxical insomnia. Additionally, the duration of spindles showed a positive correlation in patients with notable scores of extroversion (EPQR-A) and uncontrollable anxiety (PSWQ). It was noted that the frequency of spindles was the same between the healthy controls and the patients with paradoxical insomnia. This study further confirmed that certain sleep protective mechanisms are disrupted in paradoxical insomnia, as seen by the lower density of sleep spindles in these patients. Furthermore, fast sleep spindle activity can be observed with certain personality traits, as seen in those with increased levels of anxiety, therefore, leading to sleep state misperception.
Perception of being asleep or awake in insomnia patients differs from the perception in normal sleepers, and this is independent of age, sex, or laboratory versus ambulatory recording setting (31). 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 study compared REM sleep macrostructure and microstructure as well as clinical interviews between good sleepers, psychophysiological insomnia, and paradoxical insomnia. Participants underwent four consecutive polysomnograhies 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 (26).
New research has further shown that although there may be no overt changes to the overall macrostructure of sleep in patients with paradoxical insomnia, there may be changes to the underlying microstructure of sleep. Studying components, such as microarousals or cyclic alternating pattern (CAP), in patients with paradoxical insomnia are facets of microstructure that can offer more detail and convey the physiologic abnormalities that may be missed by studying overall sleep macrostructure. In a study published by Wenjing and colleagues, PSG microstructure findings of 89 patients with paradoxical insomnia were compared with 41 healthy control subjects without sleep-related complaints. There was a noticeable increase of spontaneous arousals during rapid eye movement (REM) sleep in patients with paradoxical insomnia when compared to healthy controls. This was not observed with non-rapid eye movement sleep. As REM sleep is important for subjective sleep quality and fulfilment of rest, these increased instances of microarousals and REM sleep instability can form some of the foundation for insomnia-related complaints in patients with paradoxical insomnia (27).
A study by Yoon and colleagues studied factors associated with negative and positive sleep state misperception (35). As sleep duration can be underestimated (negative state misperception) as well as overestimated (positive state misperception), sleep state misperception should fall under a “continuum of sleep perception.” The patients were divided into two groups based on the misperception index, which gives a numerical value to the discrepancy between self-reported total sleep time and objective total sleep time as measured by nocturnal polysomnography reading. A positive misperception index value (> 0) indicated underestimation of total sleep time, whereas a negative misperception value (< 0) indicated overestimation of total sleep time. The patients with a positive misperception index value were categorized into the Positive Sleep State Misperception (PSSM) group, whereas those with a negative misperception index value were placed into the Negative Sleep State Misperception (NSSM) group. Of the 150 patients, 76.7% were sorted into the NSSM group, and 23.3% were categorized into the PSSM group. It was noted that patients with negative sleep state misperception were significantly older than patients with positive sleep state misperception. Polysomnographic recording showed that the PSSM group displayed larger quantities of N3 or slow-wave sleep. Slow-wave sleep is linked with the perception of adequate sleep duration and accurate sleep time estimation. Therefore, patients with inadequate slow-wave sleep may be more prone to underestimate their sleep duration. This suggests that quality of sleep can be poorer in older individuals when subjectively measured. Addressing these misperceptions of sleep with a cognitive approach could be beneficial for these patients to correct dysfunctional beliefs surrounding sleep.
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 one 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 polysomnography recordings in insomnia patients have been explored in a number of studies. A study by Lecci and colleagues looked into electroencephalographic patterns, specifically whether spectral power changes that are not routinely present in polysomnography monitoring could explain why some patients feel awake during parts of the night and, therefore, underestimate their total sleep time, such as in paradoxical insomnia (20). Subjects underwent home sleep testing and estimated their total sleep time the next day. Patients who underestimated or overestimated their sleep time were separated from those who accurately or almost accurately estimated their total sleep time. Ten patients with paradoxical insomnia underwent supervised high-density EEG sleep recording in the laboratory to evaluate spectral power EEG changes and further distinguish the under-and overestimators from those who accurately estimated their total sleep time. The study found that those who underestimated their total sleep time had increased EEG activation over the central electrodes in both non-REM and REM sleep, which included a shift from lower to higher frequency power, thus, displaying a “wake”-like pattern. These findings were not linked to age, medications, or testing setting (home versus in lab). This increased activation in the central areas of the cortex is thought to play a role in increased feelings of wakefulness as these areas have an abundance of noradrenaline.
On the contrary, those with overestimation of total sleep time showed lower frequency power and decreased activation of EEG in REM sleep. An imprint of NREM sleep is EEG frequencies in the delta range, constituting slow-wave sleep. Oppositely, neuronal firing is represented by high frequencies, representing the brain to be in a more “wake”-like state. Beta power correlates with autonomic arousal. Therefore, the results of this study suggest that underestimation of total sleep time is linked to high frequencies and, thus, EEG activation, leading to feelings of wakefulness and underestimation of total sleep time. Conversely, those who overestimated their sleep showed increases in lower frequencies of EEG; however, this was only during the REM stage of sleep, which usually judges subjective sleep quality (20). This study suggests that slow frequency activity in REM sleep can contribute to overestimating sleep time. The findings from this study step away from generalizing misperception of sleep state and shift gears towards evidence that insomnia patients may actually be perceiving subtle changes from “sleep to wake-like brain activity,” which are not usually encompassed in the standard polysomnography montage.
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 (25). 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 (12).
Misperceptions of sleep can occur in normal healthy volunteers. 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 (08).
Another study by Te Lindert and colleagues looked at sleep state misperception accounting for night-to-night variability using seven nights of sleep diaries and actigraphy recordings of 181 patients with insomnia and 55 people without sleep complaints (30). In this study, good sleepers tended to accurately estimate or overestimate their sleep time. However, actigraphy data suggested objectively longer time spent asleep compared to subjective self-reported shorter time asleep. The data revealed three subtypes of the subjective features of sleep leading to sleep state misperception: one’s shortest reported sleep duration and the mean and standard deviation of misperception. These characteristics showed differences across days, suggesting that night-to-night variability should be considered with this diagnosis. In future studies, insomnia patients can hopefully be assigned to one of these subtypes. Further studies could also look at the response of these subtypes to cognitive and pharmacologic therapy.
Certain patient populations may be more prone to underestimation or overestimation of sleep times. Patients with posttraumatic stress disorder underestimate total sleep time and sleep efficiency compared to objective sleep measured by actigraphy (16). Patients with mild traumatic brain injury have been shown to perceive that they sleep a bit less than they actually do on polysomnography. A sample of 37 patients reported they slept 342 minutes in the lab when they actually slept 382 minutes on average (34).
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.