In this study, we analysed differences in levels of depressive signs among individuals reporting different patterns of nocturnal symptoms of insomnia. We found HADS-D scores to be lower in controls compared to insomnia cases and there were several statistically significant differences between patterns of symptoms. These differences were greater for men than women.
Individuals reporting symptoms of insomnia scored an average of 2.2 points higher than controls. This difference was statistically significant and can be considered meaningful as it is over the minimal important difference (MID) for HADS-D, estimated to be between 1.9 to 2.3 points (Chan et al., 2016). Moreover, these results are in line with the notion that insomnia is associated with depression as reported by, among others, a meta-analysis from 2016 (Li et al., 2016).
HADS-D results were statistically different also among several patterns of symptoms. Experiencing terminal insomnia alone (M = 4.1), did not differ in HADS-D score significantly from experiencing only sleep onset problems (M = 4.0). However experiencing these two symptoms at the same time produced significantly higher average HADS-D score (M = 5.0). As previously suggested (Bragantini et al., 2019), this combination of symptoms might affect sleep length more than other patterns and therefore produce more severe consequences (Vgontzas et al., 2013). In a previous article, we reported a similar results also for the anxiety subscale of HADS (HADS-A) (Bragantini et al., 2019). This suggests that experiencing sleep onset problems together with terminal insomnia reflects a more severe psychological distress than for other patterns of symptoms.
This combination of symptom may be the result of a peculiar etiopathology or of the presence of different, concomitant factors. Previous studies reported sleep onset insomnia increasing the risk for debilitating mental problems (Canivet et al., 2014) while neuroimaging brings evidence of a connection between early morning awakenings and depression (Stoffers et al., 2012; Yu et al., 2018). Speculatively, it is possible that the two insomnia symptoms and depression are connected in a consecutive and escalating manner. Early morning awakening may be a symptom of depression from the beginning due to functional changes in the OFC while sleep onset could emerge when depression exacerbate.
Surprisingly, experiencing also the third symptom, maintenance problems, did not significantly increase the HADS-D score (M = 5.2). This is consistent with the fact that maintenance insomnia has the lowest HADS-D score (3.4 points), which was the lowest score among the patterns of symptoms. However, the difference among maintenance insomnia and other single symptoms was statistically significant but not meaningful, as it was only 0.5 point with sleep onset insomnia and 0.7 point with terminal insomnia. This is in agreement with previous studies in which single nocturnal symptoms of insomnia did not differ in measure of depression (Taylor et al., 2005; Pillai et al., 2015).
Analysing the data in another perspective gave a similar view. When participants were divided into four groups according to increasing HADS-D scores, among people in the “Normal” group, 33% reported maintenance insomnia. This percentage was significantly higher than for all other groups. On the other hand, the percentage of people reporting all three symptoms of insomnia was significantly lower in the HADS-D “Normal” group (13%).
In general, maintenance insomnia seems to be a more common complaint than other symptoms (Canivet et al., 2014; Bragantini et al., 2019; Taylor et al., 2005) but is associated to lower psychological distress in this and our previous study on anxiety (Bragantini et al., 2019). Accordingly, artificially produced sleep fragmentation seems to have only a limited effect on increasing cortisol levels both at night and the morning after (Späth-Schwalbe et al., 1991; Hucklebridge et al., 2000). After an initial burst of cortisol at the introduction of sleep disruption, the individuals seems to habituate and cortisol levels decrease at following interruptions. These differences could be related to the phase in which sleep is disrupted (i.e. N-REM or REM) or to accumulation of sleep time counterbalancing the stress of the interruptions (Späth-Schwalbe et al., 1991).
Alternatively, this symptom may be the manifestation of other organic sleep disorders rather than correlate of psychological distress. Conditions such as sleep apnoea (SA), restless leg syndrome (RLS) and other disorders may disrupt sleep several times during the night (Bonnet & Arand, 2003) without leading to severe depression or anxiety.
Previous studies found that individuals reporting “combined insomnia” (i.e. sleep onset and maintenance insomnia) scored significantly higher than those with single symptoms in measures of depression (Taylor et al., 2005; Pillai et al., 2015). Similarly, in our study, participants reporting combined sleep onset and maintenance/terminal insomnia had higher scores, which was statistically significant, than respondents with only one symptom, except for terminal insomnia. This may indicate that the burden of sleep onset insomnia is somehow magnified only in the presence of another symptom. As discussed earlier, maintenance insomnia may be the result of somatic diseases. The relatively low psychological burden of these conditions could interact negatively with the one provoked by concomitant sleep onset insomnia, intensifying symptoms of depression.
In contrast to previous findings, in our sample, men had more pronounced HADS-D scores (M = 3.3). A meta-analysis from 2017 showed that women are two times more likely than men to be diagnosed with Major Depressive Disorder (MDD) and 1.6 times to have depressive symptoms (Salk et al., 2017). Even so, the inverted trend of HADS-D scores have been described before and appear to be a peculiarity of the HUNT cohort. Reasons for this are not clear, but the lack of questions about somatic symptoms of depression in HADS-D, which are more frequent in women, are hypothesized to be the cause for this unusual trend (Langvik et al., 2016).
The lack of somatic symptoms of depression in HADS-D questionnaire results in an over-representation of the anhedonic symptoms. These are typical of the melancholic depression sub-type (Fletcher et al., 2015), characterized also by sleep difficulties, specifically in the form of early morning awakenings (American Psychiatric Association, 2013). From our results we cannot confirm this notion which seems to emerge from clinical practice more than scientific literature. Future studies could deepen the knowledge on the relationship between insomnia and depression by including also measures for single symptoms of depression. This approach could bring clarity over which symptoms of depression and which symptoms of insomnia are more tightly correlated.
Strengths, limitations and future prospective
In this study, we were able to analyse all possible patterns of nocturnal symptoms of insomnia thanks to the use of data from the HUNT study. Moreover, free healthcare and strong welfare measures in the Norway reduced the confounder potential of socioeconomic factors.
The HUNT3 study sleep questionnaire presented only the three aforementioned questions on nocturnal symptoms of insomnia. Sleep length, sleep satisfaction and duration of the sleep problems, were not present in the HUNT dataset and therefore we could not produce a more defined definition of the three nocturnal symptoms of insomnia.
Even if we did not find large differences in HADS-D among people experiencing different patterns of symptoms of insomnia, the picture may be different in a clinical sample. In future studies, focusing on patients diagnosed with insomnia using tools with higher resolution may highlight these differences.
The approach of stratifying results of studies on insomnia by nocturnal symptoms could improve the knowledge on insomnia on several levels. In particular, identifying the specific psychopathological characteristics of the symptoms and integrating them with findings from sleep research could help elucidate the neurobiological mechanisms that link insomnia and depression. Moreover, identifying which symptoms of insomnia are more likely to be accompanied by severe depressive symptoms may be a useful information in the clinic. For example, patients reporting the nocturnal symptoms associated with more depressive symptoms could be monitored more closely for depression. At the same time, the therapeutic offer could also be tailored to the reported symptoms of insomnia to include interventions that target depression.