This study showed that a proportion of subjects with depressive disorder do not suffer from insomnia. On the other hand, a number of subjects with depressive disorder showed symptoms of primary insomnia and some had met criteria for both primary and secondary insomnias. It must also be noted that while primary insomnia frequently follows a depressive episode, secondary insomnia often precedes it. Importantly, age, total duration of depressive illness, number of depressive episodes, and HAM-D scores were not different between subjects with and without insomnias, irrespective of their nature – primary or secondary. Similarly, family history of person with primary or secondary insomnia did not affect the development of insomnia in that group. However, duration of insomnia – whether primary or secondary or combined – was positively correlated with total duration of depressive illness, number of depressive episodes, and duration of current depressive episode.
Primary insomnia has never been examined in depression, and hence we do not have any comparable study. Therefore, in present discussion we will focus on issues related to independent diagnosis of depression and insomnia, factors that can provide a seed for the development of primary insomnia in depression, and so on.
Whether sleep disturbance is an integral part of a depressive episode is questionable. A large epidemiological study had shown that all subjects with depression do not develop insomnia.[2
] Functional imaging of the brain during a depressive episode provides the biological basis of sleep disturbance during depression and suggests that discrete brain areas are responsible for different symptoms in a depressed patient.[14
] This is one reason why insomnia is not considered merely as a symptom of depression but considered comorbid.[9
] A number of studies support the fact that insomnia is an independent illness that co-occurs with depression. First, insomnia is known to worsen dramatically when depression reaches a critical limit, further supporting the notion that they are in fact two disorders.[4
] Second, insomnia had either preceded a depressive episode or led to a depressive episode or seen a residual symptom after treatment of depression.[2
] This is possible only when insomnia occur independently of depression; otherwise, one would have expected it to start with depression and to remit with the antidepressant therapy. Third, when depressed patients are subjected to psychological therapies for insomnia in addition to pharmacotherapy for depression, response is often better as compared with antidepressant therapy alone.[8
] This is another reason why insomnia is now considered to be a separate entity.
It is also possible that depressed subjects with primary insomnia have heightened arousal and they have behaviors that are counterproductive to sleep as compared with depressed subjects without primary insomnia. Although a direct evidence in this context is not available, but an indirect evidence may give us a clue. Earlier, it has been reported that sleep-related behavior does not differ among subjects with psychophysiological insomnia and insomnia secondary to depression and control group.[5
] In addition, subjects with psychophysiological insomnia and insomnia with depression have comparable cognitive arousal and dysfunctional beliefs regarding sleep.[5
] However, in that study, diagnostic criteria for psychophysiological insomnia and insomnia associated with depression appear to be loose and hence conclusion cannot be reached. Moreover, the sample was small, which precludes from generalization of results. On the contrary, subjects with insomnia – whether psychophysiological or ‘insomnia with depression’ often have difficulties with sleep hygiene.[5
] This perpetuates insomnia in these persons as we have seen in the present study and they suffer from combined insomnia (primary and secondary occurring together). Furthermore, depressed patients often develop cognitive distortions and their perception regarding their health, future, and relationships are negatively shaded. This has been shown in patients with myocardial infarction.[18
] We assume that some of the individuals with depressive disorder perceive their existing insomnia or depressive symptoms negatively and consequently develop primary insomnias. We have not examined sleep-related cognitive mechanisms of these patients, and this is clearly an area for further research.
Taylor et al
] had shown that depression severity was related to the number of nocturnal awakenings and the frequency of insomnia was associated with both depression and anxiety. Another study suggested that severe insomnia in depressed patients was associated with long duration of current depressive episode and higher HAM-D scores.[20
] We have not measured insomnia severity directly. However, duration of insomnia is a surrogate marker of severity and we have found its positive correlation with total duration of depressive illness, number of depressive episodes, and length of the current depressive episode.
However, this study had some methodological limitations. First, the sample size was small due to strict inclusion criteria followed in this study. In future, a study with a larger sample may be planned to unravel this issue. Second, as already mentioned, we did not assess the insomnia severity owing to absence of a rating scale in Hindi language. Third, duration of depressive episode had a large variance. A number of factors might be responsible for it: Recall bias, poor treatment compliance and consequently incomplete remission in the included subjects, inaccessibility of specialized services soon after the onset of illness, and so on. Fourth, we could not divide the insomnia subjects into three groups – those with primary insomnia, those with IDD, and a combined insomnia group – owing to the small sample size. This kind of grouping could have thrown more light on the issues addressed in the present study. Fifth, the HAM-D scores do not represent the longitudinal severity, and considering the long duration of depressive episodes in the present study, HAM-D scores do not adequately represent the magnitude of depression. Sixth, this was a cross-sectional study and we could not examine the effect of the antidepressant therapy on either of the insomnia groups. An adequate treatment of depression is thought to clarify the diagnostic confusion between primary insomnia and depression-related insomnia.[10
Nevertheless, despite limitations, to best of our knowledge, this is the first study to examine the presence of primary insomnias during depressive disorders. This may help in choosing an appropriate psychological treatment for primary insomnias early in the course of treatment of depression, and we may expect a better outcome.
In conclusion, primary insomnias are frequent during depressive disorders. They usually follow depressive symptoms. On the other hand, secondary insomnias often precede depressive illness. If insomnia remains untreated for a long time, it can lengthen the depressive illness as well as the current depressive episode and increases frequency of depressive disorders.