Nowadays, insomnia is the most widespread sleeping disorder of this century. Being one of the most common sleep disorders, insomnia can be defined as inability to maintain a good sleep hygiene. According to epidemiological studies, atleast one third of the general population suffer from sleep troubles and among this, the incidence rate of insomnia is around 6% to 10% (Daley et al. 2009). Insomnia can be viewed as either being a symptom or a syndrome; the term secondary effect or symptom can be applied in cases set off by psychiatric afflictions, diseases, intake of illicit drugs or excess of alcohol and even an aggregate of all these factors coupled with stress, anxiety or depression (Sivertsen et al. 2009). Insomnia can be evaluated using the Insomnia severity index (ISI) which based on the patients providing an assessment of the intensity of their symptoms (Bastien et al, 2001). The Pittsburgh Sleep Quality Index (PSQI) which makes use of questionnaires is a practical way to assess the sleep quality along with the troubles causing it over a certain period of time (Backhaus 2002). Insomnia is regularly linked to psychiatric disturbances such as depression. Some researches show that depression is the factor that causes insomnia while others disagree by stating that in fact, insomniacs are more prone to develop depression (Isaac & Greenwood 2011). Depression can be investigated using the Beck Depression Inventory (BDI). This literature review will be focused on defining the different types of insomnia and whether or not they are caused by depression. The flow of the bidirectional relationship between insomnia and depression will also be presented to determine which one of these two is a risk factor.
Insomnia caused by depression
Types of insomnia
Primary insomnia (PI) is the repetitive inability to commence or uphold sleep and this excludes any natural or psychotic disturbances (Backhaus 2002). This has an impact on the quality of life of the patient causing distress and unfitness both socially and physically. Woods et al. 2008 reports that PI occurs in atleast 3% of the population in the western developed nations. According to a study conducted in the general population by Morin et al. (2006), people complaining of dissatisfaction in their sleep are more like to be developing insomnia symptoms compared to those have an appeasing and pleasant sleep. Some of the symptoms that might qualify a person as being insomniacs are: difficulty to fall asleep, have trouble in upholding sleep, waking up early in the morning or have a non-regenerative sleep; these symptoms can either appear individually or in combination with one another as shown in studies by Leger et al. (2010). PI does not occur due to any other particular health conditions. It is independent of other factors.
As defined by the DSM (Diagnostic and Statistical Manual of Mental Disorders), secondary insomnia can be linked to mental disturbances, trouble to sleep owing to a medical state or stimulation by a substance. Secondary insomnia is one which is related to another mental disturbance whereby one of the chief grievances is based on the inability of start or maintains a proper sleep and that the sleep even of it is initiated is not rejuvenating at all and this spans for over a period of atleast one month. It occurs in close collaboration with other psychiatric and medical conditions whereby clinical depression is one of the main concern, this state is considered as a causative agent for insomnia.
Transient or Acute Insomnia
Insomnia can be classifies as being transient or acute is the delay of sleep disturbance expand over a period of less than a few nights or not more than three to four weeks (Fetveit et al. 2008). This type of insomnia is more likely to occur in people who have no previous history of sleep disorders but who have been exposed to some distinctive cause such as caffeine intake, nicotine or any other medications.
Chronic insomnia can be interpreted in two different ways; either as a syndrome as a whole similar to PI or as being the consequence arising from a medical ailment such as secondary insomnia or mental imbalance such as major depression, anxiety or by overdose of drugs and alcohol (de Sainte Hilaire et al. 2005). In relation to Fetveit et al. (2008), primary insomnia caters for around 25% of all the cases of chronic insomnia.
2.2 The Mechanism of how Depression causes Insomnia
Depression is a factor that triggers insomnia by causing imbalance or deficiency in terms of serotonin which is monoamine neurotransmitter. These particular serotonergic activies govern the sleep regulation. According to studies conducted in vivo by Joensuu et al. (2007), the availability of the serotonin transporter (SERT) changes at different stages of depression and this can be proved by using a technique known as SPECT (Single proton emission tomography). This decline in terms of SERT which is due to depression is mostly localised in the diencephalon in the mid part of the brain (Joensuu et al. 2007). This decline in terms of serotonin level negatively influences the sleep patterns since the serotonin is key constituent affecting both the sleep phases and mood (Buckley & Schatzberg 2010). The hypersecretion of cortisol along with ACTH is an indicator of insomnia. Changes made to the Hypothalamic-Pituitary-Adrenal (HPA) axis along with the mode of action of the hormone that predicts the start of sleep triggers the onset of depression. The hormone melatonin keeps the circadian rhythm of the body constant. Some studies show that depression causes the level of melatonin to drop which in turn causes a domino effect on the serotonin level. In people with major depressive syndromes, the concentration of cortisol is much higher than that of melatonin (Buckley & Schatzberg 2010) as shown in the graphs below which highlights the relationship between the level cortisol and melatonin.
Fig. 2 shows the phase relationship between cortisol and melatonin for healthy control compared to the one suffering from mental disorder (Buckley & Schatzberg 2010).
Yet, there are still other numerous neurobiological mechanisms like the deficiency of the monoamine neurotransmitters, excessive activity in the HPA axis, the faulty action of the gene related to plasticity and circadian genes mutation that can give an explanation on the irregularity in terms of sleep patterns in depressive patients (Benca & Peterson, 2008). This diagnosis of sleeping troubles using only polysomnography is not reliable and must be supported by functional imaging or EEG activity patterns to define the correlation between the behavioral and experimental observations (Benca & Peterson 2008).
The bidirectional flow of the relationship between insomnia and depression
Depression causes insomnia
In those people who suffer from MDD, one of their major grievances is insomnia. In the adult population, around 60% of them that fits the criteria defining MDD whine about insomnia and an average of 10% to 20% of the insomniacs show signs of MDD (Fava et al. 2006). Using multiple variances, the mean values mean response for two particular groups (insomniacs with or without symptoms of depression) responses pertaining t to BDI-II were analyzed. From the total score derived from the BDI-II, it was seen that depressed plus insomniac participants attained much higher grades on the total score scale in contrast to those with solely insomnia. A feature that highlights depression is the shift in the sleep pattern that leading to insomnia and other disturbances. Among 90% of the depressive patients are insomniacs as well according to a study by Fava et al. (2006), this clearly emphasize on the co-existence of depression and insomnia.
Insomnia triggers depression
Though not the only one, insomnia is regarded as being the main factor that has an impact on depression. Studies conducted by Pigeon et al. (2008), patients with insomnia are more likely to become depressed and remain so. From the cohort study obtained from health surveys of the HUNT-2 by Neckelmann et al. (2007), the relationship between insomnia and depression is simply based on their resemblance to one another. This defines insomnia as a state marker for depression. As established by Taylor et al. (2005) with BDI and sleep diaries to support the research, the probability of insomniacs developing depression was 9.82 times more than people without insomnia as presented by the table 1. Further, this same study states that the depression score for patients with combined insomnia is much higher compared to other studies.
Table 1. Prevalence Rates expressed using the Beck Depression Inventory Score to relate insomnia and depression (Taylor et al. 2005).
From the direct analysis of figure 3 below that 4 out of the 5 different depression statuses prevail from persistent insomnia while more than 50% of the sample did not remit or have an improvement from the condition (Pigeon et al. 2008). This proves that insomnia (persistent or intermediate) does have a significant impact on the number of people with improved condition. All these facts boil down to the fact that insomnia is certainly a major risk factor for depression.
Fig. 3 highlights the relationship of insomnia and how it is a risk factor for depression (Pigeon et al. 2008)
Different types of insomnia react differently when exposed to other factors such as MDD for example it does not in any way cause primary insomnia however in cases of secondary insomnia, it is considered as being the most crucial factors based on studies related to DSM.
This study did not really determine whether a correlation exists between insomnia and depression since they have common symptoms, yet the use of BD1-II did perceive the symptoms of depression in insomniacs.
The PSQI has a high sensitivity and specificity for
insomnia patients in comparison to healthy controls, thus
underscoring that it is a good measure for differentiating
between good sleepers and patients suffering from sleep
disturbances. Our data suggest, however, that the cut-off
score should be set to 6 in order to maximize specificity
while only modestly reducing sensitivity. In sum, the PSQI
proved to be a valuable adjunct to clinical work on insomnia
and is a useful first-line, easy-to-handle, and time-efficient
questionnaire to evaluate sleep disturbances.