*1.4.2 Objective sleep data*

*Psychopathology - An International and Interdisciplinary Perspective*

nography or actigraphy [5].

**1.2 Nocturnal panic attacks**

**1.3 Why study sleep in panic disorder?**

suicidal behavior [10].

clinician-rated assessments; the latter uses physiological measures such as polysom-

NPA are paroxysmal events characterized by abrupt awakening in a state of intense anxiety and discomfort [6]. In contrast with panic attacks that begin after waking, individuals experiencing NPA wake up in a state of panic [2]. Nocturnal panic attacks generally occur between stage 2 and stage 3 sleep and are not associated with the content of a nightmare [6]. In the majority of cases, daytime panic attacks are more frequent than NPA. However, a minority of patients primarily experience nocturnal panic attacks [7], and cases of individuals with exclusively nocturnal panic attacks have been reported [8]. Nocturnal panic attacks can be assessed using the NPA appendix from the anxiety disorders interview schedule for DSM-IV. This clinicianadministered interview thoroughly assesses NPA and includes questions relating to NPA frequency, apprehensiveness about future NPA, and avoidance behaviors.

Disturbed sleep in patients with PD is associated with greater PD severity [9]. In a recent study, researchers found that there was a significantly higher prevalence of insomnia (insomnia severity index > 8) in patients with severe or moderate PD symptoms than in patients with mild symptoms [9]. In addition to being associated with symptom severity, sleep disturbances, specifically NPA, are associated with

Not only are sleep disturbances (insomnia and NPA) associated with greater PD severity and with suicidal behavior, they are also hypothesized to perpetuate panic symptoms. Researchers have proposed that insomnia, NPA, and panic interact and reinforce one another in a vicious cycle [11]. Studies of individuals from the general population revealed that one night of sleep deprivation increases general anxiety and physiological activation [12]. In a similar experiment with patients with PD, researchers observed panic attacks in 40% of patients after one night of sleep deprivation, although none of the participants had experienced a panic attack in the prior week [13]. In the same study, none of the control participants (healthy controls or patients with obsessive-compulsive disorder) reported a panic attack the following day [13]. Although the experience of insomnia is distinct from the experience of sleep deprivation, patients with chronic insomnia can develop a sleep deficit [14]. Some researchers have therefore hypothesized that the effect of chronic insomnia could be comparable (although less intense) to the effect of sleep deprivation, resulting in increased general activation and triggering panic attacks [11]. When an individual also experiences NPA, they may develop apprehensiveness about going to sleep [15]. Apprehensiveness can result in a delayed bedtime, thereby compounding lack of sleep, increasing general activation, and potentially triggering panic [11]. Furthermore, some patients with PD tend to experience distress and even panic attacks in states of relaxation or states of decreased vigilance [16, 17]. It is hypothesized that such a reaction may occur

immediately prior to sleep in some patients and may disturb sleep onset.

Numerous studies have reported subjective sleep data from patients with PD, collected using a wide variety of psychometric tools. For example, many studies

**1.4 Unresolved issues related to insomnia in patients with PD**

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*1.4.1 Subjective sleep data*

In contrast to subjective sleep data, objective data is collected in a standardized fashion, generating results that are comparable across studies. However, despite greater uniformity in assessment, the literature on objective sleep data has yielded contradictory results. For example, some authors have reported poorer sleep efficiency in patients with PD in comparison to healthy controls [19, 26], whereas others report no difference between groups [27]. Similarly, inconsistent results have been published about slow-wave sleep (stages 3 and 4) and REM sleep latency [6, 22, 26–31]. The wealth of conflicting data is confusing and can lead to erroneous conclusions. For example, if two studies report a difference between PD and control groups in a given variable and two others report no difference, readers tend to conclude that no substantial difference exists, which may not be the case [32]. Among possible explanations, inconsistencies may be partly attributable to a lack of statistical power: indeed, polysomnographic studies tend to have small sample sizes due to their costly and complex nature.
