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