Measurements and procedure

Clinical evaluation of RLS symptoms and severity as well as general health, quality of life, and sleep quality assessments were implemented. Subjective and objective measurements were included to assess sleepiness. Current levels of subjective sleepiness were measured before assessing objective measures of sleepiness. The entire assessment took approximately 2 h and was usually performed in the mid-afternoon. Sleep–wake activity was monitored via actigraphy during the night prior to the assessment. All participants were instructed to take sufficient time for rest (usually equivalent to time in bed; minimum 6 h) to avoid any unusual sleep deprivation in the night prior to the assessment.

Essential criteria of RLS.

The presence of essential diagnostic criteria in accordance with the IRLSSG guidelines (Table 1) was assessed by clinical interview.

International Restless Legs Severity Scale.

The IRLSSG questionnaire (IRLSS) consists of ten questions regarding the subjective severity of RLS symptoms, ranging from 0 (none) to 4 (very severe). A score above 10 is considered moderate, a score above 20 points severe, and a score above 30 very severe [18].

Pittsburgh Sleep Quality Index.

The Pittsburgh Sleep Quality Index (PSQI) assesses the quality of sleep and awakening as well as somatic symptoms. A global score above 5 indicates poor sleep quality [4].

36-Item Short Form Health Survey.

The 36-Item Short Form Health Survey (SF-36) includes eight scales to assess physical and mental health. Scores range from 0 to 100, with higher scores representing a better health status [3].

Epworth Sleepiness Scale.

The ESS assesses the likelihood of dozing off or falling asleep in different everyday situations, in contrast to feelings of tiredness. The ESS [8] is a self-administered eight-item questionnaire (ratings of 0–3 for each item; total score of 24). Based on normative data from the German population, a score greater than 10 is considered “clinically suspicious,” and a score greater than 12 is considered “clinically relevant” [12].

Tiredness symptom scale.

In the tiredness symptom scale (TSS), patients identify 14 physiological and psychological symptoms with 0 (no) or 1 (yes). Higher scores indicate increased levels of subjective sleepiness [15].

Stanford Sleepiness Scale.

In the Stanford Sleepiness Scale (SSS), patients rate their current state of alertness on a seven-point Likert scale ranging from 1 (very alert) to 7 (unable to stay awake) [7].

Mackworth Clock Test.

The Mackworth Clock Test (MCT; VIGIL S1, Schuhfried GmbH, Mödling, Austria) objectively assesses sustained attention using a monotonous task. Participants are instructed to observe a monitor on which a dot follows an orbit of 32 small circles, jumping every 1.5 s, and to press a button every time the dot skips one of the circles. These critical stimuli appear in a randomized order with a total of 100 skips over the 25-min test duration. Several variables are measured, including mean reaction time (RT), number of lapses, and number of false alarms [11].

Psychomotor vigilance task.

The psychomotor vigilance task (PVT) objectively assesses attention using reactions to frequent simple visual stimuli presented on a screen during a 10-min test period. Participants are asked to press a button whenever time starts being counted on the screen [5]. The main outcome measure was the mean RT.

Pupillographic sleepiness test.

In the pupillographic sleepiness test (PST), the diameter of the pupil is measured during 11 min in total darkness. The mean variation of pupil size per unit time is expressed as the pupillary unrest index (PUI [mm/s]). A higher PUI indicates sleepiness [23].

Sleep–wake activity monitoring.

In addition to the above tests, actigraphy in ambulatory settings (miniaturized accelerometers; Motionlogger; Ambulatory Monitoring, Inc., Ardsley, NY, USA; recording length, 16 h, epoch length, 2 s, zero-crossing mode) were performed to record sleep–wake activity and motor activity, such as periodic limb movements, the night before the assessment. Actigraphy and sleep diary data were mainly used to check for adherence to the study protocol and sufficient rest. Sleep–wake parameters and periodic limb movements in sleep (PLMS) were approximately determined using the standard monitoring software used in the study hospital (Action4®; Ambulatory Monitoring, Inc., New York, NY, USA). Full attended cardiorespiratory polysomnography was performed (according to American Academy of Sleep Medicine guidelines) only when necessary in individual cases to rule out other sleep disorders, which was the case in 3 patients (data not reported here).

Participants

The patients in this study were recruited from a pool of potential candidates for intended pharmacological studies on RLS symptoms. A total of 29 patients with idiopathic RLS were included. They were newly diagnosed at the Center of Sleep Medicine, Department of Psychiatry and Psychotherapy, University of Regensburg, according to the IRLSSG diagnostic criteria [1] and were treatment naïve. Exclusion criteria were strict and included (i) history of, current use of, or dependence on substances according to the Diagnostic and Statistical Manual of Mental Disorders‑5 criteria for substance use disorders (alcohol, hypnotics, or other substances except nicotine); (ii) severe comorbid psychiatric, neurological, or medical disorders that could confound the study results; (iii) use of psychoactive substances or medications that could affect RLS symptoms, sleep quality, or wakefulness (e.g., amphetamines, methylxanthines, sedatives, hypnotics, antidepressants, antihistamines, neuroleptics, beta-blockers); and (iv) excessive tobacco smoking (more than 15 cigarettes/day). Due to the a priori inclusion and exclusion criteria of the subsequent pharmacological studies (e.g., need for effective contraception), recruitment was biased toward men. Eight participants were women and 21 were men, with a mean (±standard deviation) age of 52.9 ± 13.0 years. The age- and sex-matched control group consisted of 31 healthy adults with a mean age of 51.0 ± 13.3 years (11 women, 20 men). For the control group, participants were recruited among hospital staff and via flyers and newspaper ads. All participants provided written, informed consent.

Study design and data analyses

The study followed a between-subject, single-measures design, featuring a group of patients with RLS and a healthy control group. The study was approved by the local ethics committee of the University of Regensburg and fulfilled the principles of the World Medical Association Declaration of Helsinki.

Results from standardized test instruments were compared between patients and healthy controls. As the data were not normally distributed, Mann–Whitney U tests were conducted. Statistical analyses were performed using SPSS software (version 25.0; IBM Corp., Armonk, NY, USA). Results with p-values < 0.05 were considered statistically significant. The measurement of quantitative actigraphic data, including PLMS indices, sleep duration, and sleep latency, was limited due to methodological issues. Therefore, PLMS and sleep–wake activity data were evaluated qualitatively by visual inspection and were not used for statistical comparisons.