Insomnia is a primary disorder or a secondary disorder induced by psychiatric or physical disease, most notably in major depressive disorder (MDD) (Riemann et al., 2020). Previous studies have reported that co-morbidity between insomnia and MDD enhances the severity of depressive symptoms, decreases the rate of depression remission (Mason et al., 2020), and raises the frequency of residual symptoms (Matcham et al., 2019), contributing to poorer outcomes for both disorders (Gallo et al., 2020; Hertenstein et al., 2019).

Some research have revealed that MDD patients exhibit sensorimotor cortical abnormalities (Chen et al., 2021a). MDD patients have diminished visual contrast sensitivity, poor auditory processing of non-speech stimuli, and higher pain tolerance to external sensory stimuli (Thompson et al., 2016). In addition to these changes in external sensory inputs, MDD has been found alterations in interior sensation processing and psychomotor retardation, such as decreased pain tolerance to internal sensory stimuli (Thompson et al., 2016) and decreased accuracy in heartbeat detection (Pollatos et al., 2009).

Sleep is a reversible behavior that is both perceptually disengaged and environmentally sensitive. Poor nocturnal sleep correlates with hypersensitivity to external sensory stimuli, such as tactile, visual or auditory input (Bastien et al., 2013). Numerous neuroimaging studies have suggested abnormalities in sensorimotor-related brain regions in patients with insomnia. A resting-state functional magnetic resonance imaging (rs-fMRI) study revealed that functional connectivity density in the inferior temporal gyrus increases with insomnia symptoms (Engel-Yeger and Shochat, 2012; Gong et al., 2020). Another rs-fMRI investigation on insomniacs has discovered insula damage related to interoceptive sensation (Chen et al., 2021b; Wu et al., 2021) and elevated insula regional homogeneity values in patients with insomnia (Wang et al., 2016). Liu et al. (2018) discovered a high amplitude of low-frequency fluctuations (ALFF) of the anterior insular in individuals with high insomnia; increased insular functional connectivity in the salient network (SN) (Chen et al., 2014). In addition, our previous work also discovered that MDD patients with insomnia problems have functional connectivity abnormalities in the visual-related region (Tao et al., 2018). However, neuroimaging evidence regarding patients with MDD and insomnia remains limited.

Notably, patients with insomnia clinically present with a variety of symptoms, including difficulty falling asleep (DFA), insubstantial sleep (excessive and prolonged awakening), difficulty maintaining sleep, such as early awakening (EA), superficial sleep (lack of deep sleep), DFA again after waking during the night and sleep deprivation (Morin et al., 2015). However, these manifestations have been studied together as symptoms of insomnia in the past. However, the wake-to-sleep and sleep-to-awake have different dominant electroencephalography (EEG) activities. DFA often exhibits delayed circadian rhythms, and EA is a manifestation of the advanced circadian phase. Clinically, the two symptoms do not always coexist; different patients suffer from various core insomnia symptoms (Zhang et al., 2022). Unfortunately, it is unclear whether the pathogenesis of DFA or EA in MDD differs; the mechanisms underlying brain functional activity between the two symptoms and their relationship with the sensorimotor cortex remain unknown.

Therefore, we presumed that the mechanisms of brain functional activity in the specific insomnia symptoms (DFA and EA) are different; the insomnia symptoms in MDD are associated with abnormalities in sensorimotor function. This study used ALFF analyses of rs-fMRI data to examine further the neuroimaging features of DFA and EA in MDD. ALFF has been found to represent simultaneous local neuronal activity (Chen et al., 2022; Zuo et al., 2010), making it a suitable tool for studying changes in disease-related regional spontaneous activity strength under rest in a range of mental diseases (Liu et al., 2013).