In a sample of psychiatric inpatients experiencing a severe depressive episode, the majority (75.7%) experienced insomnia. The symptom complexes of fatigue and daytime sleepiness, which were seen in 66.7 and 25.3% of the patients, respectively, should be distinguished from insomnia. While reports of insomnia can be considered typical and common in the diagnosis of depression, daytime sleepiness is not a classic depressive symptom.

The prevalence of 25.3% for increased daytime sleepiness (ESS scores > 10; 16.7% for scores >12) in the patient group of the current study was higher than in the general population, but only slightly. In a larger German study with more than 9000 participants, 22.7% had clinically suspicious ESS scores (> 10), but participants were not excluded due to pre-existing diseases [30]. When comparing the prevalence of EDS in inpatients with depression to that of healthy controls, it is important to consider that inpatients in psychiatric hospitals typically follow a consistent daily routine. In contrast, population-based studies inherently involve sleep deprivation related to lifestyle factors. In our study, both ESS score cutoffs (> 10 and > 12) were selected following the normative study by Sauter et al. [31] that revealed prevalences of 15 and 5%, respectively, in 239 healthy subjects. However, the studied sample did not represent the true general population.

A similar observation was made by Hawley et al. when comparing psychiatric patients with a normative population [18]. Although psychiatric inpatients more commonly experienced daytime sleepiness than healthy controls (34% vs. 27% with ESS ≥ 10), this finding must be interpreted considering the relatively high prevalence in the unselected control group. Some studies have shown daytime sleepiness prevalences of more than 50% in patients with depression [8, 20], but these studies may have introduced selection bias while drawing patient samples from sleep clinics.

In the present study, there was no significant correlation between daytime sleepiness and the severity of depression, and previous studies have also revealed heterogenous findings. In general, sleepiness or even EDS cannot only be attributed to depression but may have other causes [7].

EDS is often indicative of hypersomnolence-related sleep disorders such as comorbid sleep apnea [11, 15]. In fact, 18.2% of patients in the present study showed polysomnographic evidence of sleep apnea syndrome, with significantly elevated ESS scores, mean age, BMI, and additional somatic diagnoses. The prevalence of sleep apnea syndrome was thus higher than in the general population, although another study in psychiatric inpatients showed an even higher prevalence of 23.8% [5]. In that study, however, the diagnosis was not made using PSG but using an eight-channel apnea screening, which could explain the higher prevalence.

In the present patient population, 18.2% had sleep apnea, with 80% of these patients being diagnosed for the first time during their hospital stay. Compared with the general population, with a prevalence of 2–7% [1], the prevalence of sleep apnea was significantly higher in inpatients with depression in the present study. These results were consistent with studies showing prevalences of 19.8–36.8% [33]. Although the frequent occurrence of OSAS in depression has been known for some time, SRBDs remain presumably underdiagnosed in psychiatric inpatients [5]. There is strong evidence for frequent comorbidity between SRBDs and psychiatric disorders, particularly depression. Therefore, screening procedures for SRBDs (i.e., evaluating subjective signs of SRBDs such as snoring, observed apneas, non-restorative sleep, and impaired daytime functioning; polygraphy as an objective screening measure) should be included in the routine work-up of psychiatric hospitals.

The present study also found a previously unknown specific sleep disorder in 16.1% of patients using sleep history and sleep-specific questionnaires. In addition to the anamnesis of typical risk factors and symptoms, the ESS can be helpful in diagnosing sleep apnea or other EDS-related sleep disorders such as RLS. Clinically suspicious ESS scores were seen significantly more often in patients with undiagnosed sleep apnea, consistent with previous studies [5, 19]. Although Sauter et al. recommend a cutoff value of 12 [31], the commonly used cutoff value of 10 proved to be a better choice in the current study. The ESS is widely used, and scores may correlate with the severity of sleep apnea [24]. A study of inpatients with psychiatric disorders found a positive correlation between the occurrence of sleep-related breathing disorders and the risk factors of age, BMI, male sex, and ESS scores [5]. Treating sleep apnea with CPAP can reduce tiredness and fatigue and improve energy levels [9]. However, epidemiological studies have shown that SRBDs are not consistently associated with impaired daytime functioning or EDS [23], and patients may also underestimate their own daytime sleepiness [6]. Thus, in line with the present finding that many newly diagnosed patients did not have daytime sleepiness (57.1%), the ESS is too nonspecific to be used as a screening tool for SRBDs, and should only be used in combination with other screening methods.

In psychiatric settings, clinicians and psychologists should be aware of the typical symptoms and risk factors for sleep apnea or other sleep disorders and suggest further evaluation when appropriate [3]. The ESS should be used routinely in combination with other tools due to its limited sensitivity (42.9%). PSG is particularly indicated in patients with refractory depression and sleep apnea and treatment of sleep apnea can positively affect both depression and general health [17]. Thus, treating SRBDs with CPAP can diminish depressive symptoms [29], sleepiness [16], tiredness, and fatigue, and improve energy levels [9].

The present study has some limitations. Some patients did not complete both the ESS and FSS, and the missing data could not be retrieved. It remains unclear whether the wording used in the German translation of the FSS [34], which uses “tiredness” instead of “exhaustion,” influenced the distinction between fatigue and daytime sleepiness, since tiredness is a very broad semantic concept. It is important to note that the study lacked a control group, such as inpatients of an orthopedic ward, to directly compare the prevalences of EDS and SRBDs. Further, the majority of patients in this study group were taking psychiatric drugs during the study period. Daytime sleepiness and fatigue may occur to some extent as side effects of medications or other diseases, and the causal effects of this remain unclear. However, due to the small number of exclusion criteria, the risk of selection bias was low.

This study has highlighted a number of issues that still need to be addressed. Considering that insomnia is a major risk factor for the development and maintenance of depression, the question arises of whether daytime sleepiness may also have such an influence. Longitudinal studies examining the effects of daytime sleepiness on the progression of depression are needed.