This article was incredibly insightful because I suffer from sleeplessness constantly and this in turn effects my mood. When I am somewhat rested, I feel more upbeat and positive, as well, I am able to tackle the day. Though, when I've had a sleepless night, I have such a hard time motivating and feel depressed. It totally makes sense that they would go hand-in-hand as this article discusses;
"Without a doubt, sleep and depression are correlated. Sleep is a natural process that is absolutely necessary for good mental health. In medical parlance, there is a high degree of comorbidity between mood disorders (particularly depression) and sleep disorders but the direction of causality can go either way. In other words, sometimes the sleep disorder is caused by the depression and sometimes the depression is caused by the sleep disorder. But before we go further, let's examine depression and sleep individually.
The American Psychiatric Association (APA) has a rather comprehensive way to define depression using their Diagnostic and Statistical Manual of Mental Disorders (DSM). But to the layman, we can all understand depression as a debilitating low mood accompanied by loss of self-esteem and loss of enjoyment in activities that once brought pleasure. Depression varies in both intensity and type. For example, it can exist on its own (unipolar depression) or it can exist as part of a person's bipolar disorder.
Similarly, sleep disorders are also officially recognized and categorized. Sleep is a complex process that is divided into several stages and is crucial for a person's well-being. In today's stressful world, it's not uncommon for the natural sleeping process to be thrown off balance. Essentially, any sleeping pattern that is not only abnormal, but also severely disrupts a person's normal day-to-day functioning can be considered as a sleep disorder. Broadly, sleep disorders can be categorized as hypersomnia, sleep apnea, circadian rhythm, insomnia and parasomnia. There are also other sleep disorders and some that are not specifically specified ('not otherwise specified' or nos for short).
So how do these two seemingly different set of disorders affect one another? Well, let's take the example of a person having unipolar depression. Occasionally, a person having depression would have hypersomnia, that is excessive sleeping or feelings of sleepiness. There have been some theories that have been put forward to explain this, but it's quite easy for anyone to understand this phenomenon. A person going through a major depressive episode would often be utterly drained of energy and motivation. The body's desire to sleep is simply a reflection of a lack of energy. Yet, there are some who would argue that it's not just a lack of energy but actual exhaustion. This is because the depressed patient will often have negative self-talk repeating endlessly in his or her mind, draining all mental resources. While this sometimes cause the patient's body and mind to collapse and seek refuge in sleep, many patients find that it's impossible to sleep with such constant thoughts automatically running in one's mind. As a result, a large number of those with depression have insomnia.
Now that we've had a quick glance at how depression can cause symptoms resembling sleep disorders, we turn to how sleep disorders themselves can cause depression. Sleep disorders, by themselves, do not usually cause depression. But they can disrupt life enough to trigger depression. For example, a person with chronic insomnia will likely always feel fatigued and agitated. Apart from the resentment this brings directly, it can also cause a person to do poorly at work or school, be short with loved ones and in general affect one's life negatively. Over time, the person may come to feel powerless and helpless and this opens the door to depression. On a more biological level, chronic insomnia can cause certain nutrients to be depleted in the body or cause a failure to repair damages in the body (normally done during sleep). This, in turn, can directly affect the person's mental functioning.
Of course, in reality, it is not always easy to tell the direction of causality between sleep disorders and depression. The medical health practitioner may choose to focus on one or the other or simply treat both problems concurrently. But the situation becomes even more tricky in the case of the bipolar (manic depressive) patient. This is due to the relationship between mania and sleep deprivation. The patient needs to be especially certain that any medication he takes does not cause sleep deprivation as this can trigger a manic episode which is likely to be followed by depression.
Regardless of the exact nature of the relationship between sleep, sleep disorders and depression though, the best precaution one can take is to practice both good sleep habits/routines and prevent depression before it can occur. Simple habits like regular exercise, having a regular sleeping time, eating nutritious foods and adopting a healthy mindset all go a long way to preventing depression and sleep disorders before they can get a hold of you and swallow you in a vicious cycle of sleeplessness and misery."
Source: Helium, by Rahman Majere