One of the most prevalent mental disorders nowadays is depression. The epidemic has resulted in a dramatic rise of 28% in depression worldwide, according to the report of the 75th World Health in 2022 (Director-General's report to Member States at the 75th World Health Assembly – 23 May 2022 (who.int)). Additionally, depression increases the chance of both suicide and disability. Patients who exhibit these symptoms report feeling unenthusiastic, thinking slowly, withdrawing from social situations, and paying more attention to themselves. Along with high rates of depression, sleep disorders are often a consequence. According to epidemiological estimates, between 50% and 90% of depressed people experience sleep problems, and these two conditions frequently co-occur(Steiger & Pawlowski, 2019). To comprehend the probable physiological and psychological processes of depression, it is therefore beneficial to examine depression in patients with concomitant insomnia.
In the DSM-5, sleeplessness is included as a depressive symptom (Psychiatry.org - DSM). Interestingly enough, sleep deprivation (SD) is now one of the treatments for depression, about 45% -60% (Geoffroy & Palagini, 2021) of patients with depression respond positively to this therapy. The antidepressant impact of SD has been explained by the two-process model of sleep regulation, the synaptic plasticity hypothesis, the biological hypothesis of the HPA axis, and proinflammatory cytokines. Additionally, circadian rhythms may be linked to hormones, cytokines, and neurotransmitters that are involved in the pathophysiology and management of depression (Kurczewska et al., 2021). The suprachiasmatic nucleus, a sophisticated biological clock in mammals, regulates its secretion. Circadian rhythm disturbances could be a prodromal sign of mood disorders. The effectiveness of sleep deprivation in treating depression is at odds with the co-morbidity of insomnia and depression. In conclusion, people with depression and insomnia may have a unique circadian rhythm and visual network. Furthermore, these participants are distinct from those who are simply depressed. To examine the relationship between insomnia and depression, we further split depressed patients in this study into those who had symptoms of insomnia and those who did not, as well as creating a healthy control group.
The EEG microstate is a complement and extension of magnetic resonance imaging research because it can comprehend and assess the dynamic state of the entire brain network at the millisecond level (Khanna et al., 2015). The microstate, which is thought of as a reasonably stable spatial stationary piece of the discontinuous instantaneous scalp potential, is a quasi-stable environment created by the instantaneous topographic map of EEG(Lehmann et al., 1987). The relatively steady microstate in space provides a temporal advantage as a result of the benefits of high temporal resolution of EEG signals (Lehmann, 1990). It is believed that microstates are crucial elements of human mind-consciousness processes because they depend on what has just occurred in the brain, so they (Lehmann, 1990; Lehmann et al., 1987). In addition, the resting-state brain network is a structured area of activity in the brain, and changes in its neurodynamics throughout time may be a key indicator of both healthy brain function and mental illnesses (Fox & Raichle, 2007). Microstate thus represents the dynamic actions of a vast brain network in addition to being a neurophysiological measure. Patients with depression may experience various global brain dynamics changes due to Insomnia. As a result, we think of conducting our analysis while the brain is at rest.
In previous spectral studies of resting-state EEG in depression, the activity of α, β, and θ bands in the occipital parietal lobe region increased in depressed patients, suggesting that depression may affect how the brain processes emotions. The α rhythm is considered to be a reliable indicator of depression (Dolsen et al., 2017). And, α asymmetry is primarily present in the frontal lobe, which might predict specific symptoms and how a treatment will turn out (Van der Vinne et al., 2017). Furthermore, γ may be related to emotional fluctuations, a large number of γ may help with emotional stability, and may even be used to foretell the effects of depression (Fitzgerald & Watson, 2018). As for EEG microstate, previous studies on depression mainly focused on severe depression or bipolar disorder. A large number of studies have found that there are aboriginal abnormalities in the indicators of EEG microstate in patients. However, the direction of variation of the microstate anomaly parameters varies inconsistently between studies. The presence of microstate A/B was associated with the intensity of depressive symptoms in a study of individuals with bipolar illness and depression (Damborska et al., 2019). All of the temporal parameters for microstates A through D may be abnormal for people with major depressive disorders (MDD) (Lei et al., 2022; Murphy et al., 2020; Yan et al., 2021). Following therapy, the occurrence and duration of microstate B decreased, whereas the duration and contribution of microstate D increased (Lei et al., 2022). In a recent study on subclinical depression, the occurrence of microstate B was higher, while the duration and contribution of microstate C were significantly lower than those of the non-subclinical depression group(Zhao et al., 2022).
The resting-state EEG study of insomnia showed that during daytime awakening, the power of β, θ, and γ bands of insomnia patients increased, and drowsiness and cortical hyper wake coexisted; the increased EEG activity (such as θ, α, and σ rhythm) of NREM during sleep, which is consistent with the hypothesis of the 24-hour hyperarousal (Zhao et al., 2020; Zhao et al., 2021). There are relatively few studies on the microstate of insomnia. Wei and his colleague discovered that insomnia resulted in a shorter average microstate C length and a higher frequency of microstate D (Wei et al., 2018). It could be associated with the improvement of body awareness and inner sensory sensitivity in insomniacs. Additionally, there was a significant correlation between sleep latency and sleep efficiency and the duration of microstate C (Ding et al., 2022).
A continuum model of abnormality may instead represent the border for mental health issues, according to mounting data. Research on subclinical depression and insomnia is essential for the prevention, detection, and treatment of depression and insomnia due to the existing huge and concealed mental sub-health population. We hope to learn more about the dynamics of the brain networks involved in subclinical depression and insomnia. Therefore, the purpose of this study is to investigate the differences in EEG microstates between healthy individuals in the resting state and subclinical depression, subclinical depression, and sleeplessness. We make the following assumptions: first, in terms of circadian rhythm, subjects with subclinical depression with insomnia have more nocturnal patterns and are more likely to be sleepy during the day; secondly, there are significant differences in the microstate time parameters of CI and CFD; thirdly, microstate time parameters of subjects correlate with the severity of symptoms, such as the scores of depression and insomnia.