A meta-analysis was conducted on the included studies that reported sleep disturbances/disorders in patients with FND. We found that in patients with FND there were a significant proportion of individuals who experienced some form of sleep disorder. Further analysis was done to establish the possible link between sleep quality and FND by analysis studies that reported WASO and ESS score. However, due to the limited studies reporting this information no significant results were obtained.

To our knowledge this is the first systematic review to address the presence of sleep disorders in patients with FND. Although there is a well-established relationship between sleep and psychiatric disorders [15, 16], there are very few data on sleep in FND. Some small studies (n < 20) have reported reduced subjective sleep quality as well as increased rapid-eye-movement sleep duration during polysomnography in patients with Psychogenic Non-epileptic seizures (PNES) [17, 18]. While the results slightly vary between studies; our study found that 58% of patients reported sleep disorders. This is similar to a recent study that found 46.7% of patients with FND experience sleep disturbances [19].

Of the 9 studies included 5 focused on patients with PNES. Latreille et al. (2018) compared sleep-wake patterns in a prospective observational study in PNES and epilepsy patients. Twenty-seven subjects were included in the study, of which 17 had PNES, and 10 were diagnosed epilepsy. Compared to epilepsy controls, the PNES patients showed increased latency of sleep onset (on average about 30 min longer than controls). Otherwise, both groups had a similar sleep architecture. However, the PNES patients subjectively had poorer sleep quality measured by Pittsburgh Sleep Quality Index (10.8 ± 5.1 versus 5.8 ± 2.9; p = 0.01; higher score indicates the worse quality of sleep) and met the clinical criteria for insomnia more often than patients with epilepsy (50% vs. 10%, p = 0.05) [20]. Compared to non-FND patients with FND were more likely to have mild to severe sleep changes, mostly shorter sleep length by up to 1–2 h, and difficulty falling asleep again. These changes in sleep patterns were linked with lower quality of life. These results propose that sleep disturbance is a more pronounced problem in PNES than in epilepsy [20, 21].

Popkirov et al. (2019) examined the occurrence of sleep disorders in a group of 22 PNES patients and 44 epilepsy patients. However, no significant differences were found between the two groups; in patients with epilepsy only three cases of mild or moderate obstructive sleep apnoea–hypopnea syndrome (OSAHS) (7%) and four cases of sleep-disordered breathing (SDB) (9%). Most of the patients were overweight or obese. In 22 patients with dissociative seizures, two (9%) had mild SDB, two (9%) had mild OSAHS and one (5%) had moderate OSAHS [22]. Similar results were obtained by Sivathamboo et al. (2019), Moderate to severe SDB was observed in 26.3% (67/255) of patients with epilepsy and 29.0% (27/93) of patient with PNES. Following adjustment for confounders, pathologic daytime sleepiness predicted moderate to severe SDB in epilepsy (odds ratio [OR] 10.35, 95% confidence interval [CI] 2.09–51.39; p = 0.004). In multivariable analysis, independent predictors for moderate to severe SDB in epilepsy were older age (OR 1.07, 95% CI 1.04–1.10; p < 0.001) and higher body mass index (OR 1.06, 95% CI 1.01–1.11; p = 0.029), and in PNES older age (OR 1.10, 95% CI 1.03–1.16; p = 0.002) [23]. Similar results were also obtained in patients with Functional Motor Disorder (FMD). Nepožitek et al. (2023) found that 23/37 (62%) of patients with FMD experienced sleep disturbances; 35% having restless legs syndrome; 49% obstructive sleep apnoea; and 8% periodic limb movements in sleep; however, the presence of these disorders was not correlated with subjective sleepiness. Patients with FMD with self-reported sleepiness reported higher fatigue (p = 0.002), depression (p = 0.002), and had longer sleep latencies in the MSLT (p < 0.001) compared to the patients with central hypersomnia [24]. Higher Wake After Sleep Onset (WASO) rates are a marker of sleep disruption and the Popkirov et al. paper highlighted this fact. WASO observed in patients with Functional seizures may not be coincidental. Research indicates a potential causal link between sleep disturbances and dissociative experiences [25]. Experimental studies have shown that acute sleep deprivation can increase dissociative symptoms in healthy individuals, as measured by self-report and cognitive tasks [26]. Insomniac patients also tend to score high on the Dissociative Experiences Scale, with these scores correlating with specific EEG findings [27]. The study by Bregman-Hai and Soffer-Dudek (2023) provides further support of this relationship between poor sleep and dissociation. The study found that poor sleep quality and posttraumatic symptoms (PTS) are independent pathways leading to dissociation and disruptions in the sense of agency. Specifically, individuals with low levels of PTS exhibited a stronger relationship between poor sleep (measured by WASO) and dissociative experiences, whereas this relationship was not observed in individuals with high levels of PTS. This suggests that sleep disturbances might contribute to dissociative experiences primarily in those with less severe posttraumatic symptoms via an independent pathway [28].

On the other hand, this opens up the possibility that targeting sleep disruption might be a therapeutic strategy to improve dissociative symptoms. Past research has shown that improving sleep quality in psychiatric patients has been associated with a reduction in dissociative symptoms [29]. However, we need a larger prospective study to explore this possibility in FND population.

Our study has several limitations. The presence of sleep disorders in the presence of FND was measured by the proportion of patients experiencing sleep disorders in each study. Due to this the heterogeneity (I2 = 89%) of studies included were high. In order to focus on the effect of sleep disturbances in FND a further analysis was done based on the ESS score and WASO time; however, due to only limited studies reporting this data no significant results were obtained. We also excluded full-text papers not available in English, restricting paper eligibility.