See Psalm 42 concerning depression.
Depression is a mental illness  in which people feel sad for no reason at all or simply find it difficult or impossible to deal with the normal things of life. Pessimism is very often the cause of depression.
Depression afflicts devout Christians at a lower rate than the non-religious, or moderate Christians. But roughly 15% of even devout Christians battle depression, perhaps due to a lack of realization of Bible verses that combat it or liberal distortions of those verses in common translations.
Severe depression affects approximately 10% of people. The feeling of depression is something that many people experience from time to time  and can even help individuals to recognize and deal with problems in their lives. In clinical depression, or bipolar disorder, the feeling of depression is worse, goes on for longer and makes it difficult or impossible to deal with the normal things of life. Although depression can be triggered by life events such as childbirth (see Postpartum depression) or the loss of a loved one due to death, sufferers also often describe feeling sad for no reason at all.
Clinical depression can manifest itself in many different ways.
- Feelings of unhappiness that don't go away
- Wanting to burst into tears for no reason
- Loss of interest in things
- Being unable to enjoy things
- Feeling restless and agitated
- Loss of self-confidence
- Feeling useless, inadequate and hopeless
- Feelings of guilt
- Feeling more irritable than usual
- Thoughts of suicide.
- Inability to think positively or hopefully
- Great difficulty in making even simple decisions
- Poor concentration or lack of concentration
- Significant, unintentional loss of appetite and weight
- Difficulty in getting to sleep
- Waking earlier than usual
- Sleeping too much
- Feeling exhausted
- Loss of libido
- Difficulty in starting or completing things, even everyday chores
- Crying a lot or feeling the need to cry, but not being able to
- Social isolation/withdrawal
Types of Depression.
Depression is characterized by many different types, including;
- Major depression.
- Dysthymia and chronic depression.
- Seasonal effects or weather affected depression.
- Psychotic depression.
- Bipolar depression.
Here the person goes into deep despair and has a loss of hope. Symptoms of this depression interfere with the person’s ability to work, study, sleep, eat and relish activities. It can occur only once, but it often happens many times in a lifetime.
Dysthymia and Chronic Depression.
Dysthymia is referred as long-running depression. It is not a severe form, but the symptoms can last for several years. People who suffer from dystopia are usually able to act normally but always seem unhappy. The condition of dysthymia is different from major depression. The symptoms of dysthymia are less than major depression. In adults it last for at least two years and one year in children or adolescents.
Seasonal Effects or Weather Affected Depression.
Weather-affected depression appears at the same time every year. Usually it starts in spring or winter and ends at the beginning of spring or summer. A unique form of weather-affected depression is known as summer depression. It starts at the beginning of spring or summer and ends in spring.
People who suffer from it, have major symptoms such as sadness, irritability, lack of interest in usual activities, running away from social activities and decreased focus etc.
About 25% of people those are hospitalized for mental illness are suffering from psychotic depression. However, people have face hallucinations in this condition. Sighted or listening to things that are not actually or delusional, which also cause irrational thoughts and symptoms of fear.
Here the mind is constantly depressed or very happy for several weeks or months. The mind of the victim is alternately in two different and opposite stages. This disease shows a sudden change in human behaviour. Sometimes the patient is very happy and sometimes very depressed.Depression: Types, Symptoms and Diagnosis
See also: Subclinical (psychology)
According to the Encyclopedia of Human Behavior: "Subclinical depression usually refers to an elevation of depressive symptoms that usually do not warrant treatment but that may nevertheless interfere with an individual's ability to function effectively."
The two best-known theories are, first, that depression is caused by a chemical imbalance in the brain; second, that gloomy thinking causes depression. According to the second theory, it is the depression that causes the chemical imbalance (not the other way around).
- The cognitive theories of depression are among the most clearly articulated and well-supported models of depression. According to these theories (e.g., Abramson, Metalsky, & Alloy, 1989; Nolen-Hoeksema, 1991), some individuals have a cognitive vulnerability that interacts with stress to produce depression. Specifically, people are vulnerable to depression because they have a tendency to generate interpretations of stressful life events (and dysphoric moods) that have negative implications for their future and for their self-worth. 
- National Institute of Mental Health
- Royal College of Psychiatrists
- WebMD Depression Health Center
- National Suicide Prevention Lifeline
- "Depression is a mental illness that can affect mood, cognition, and physiological functioning. Each year about 21 million American adults (nearly 10% of the population) suffer from depression or a related illness. Robins, L.N. & Regier, D.A. (1990). Psychiatric Disorders in America: The Epidemiologic Catchment Area Study. New York: The Free Press.
- Learned Optimism by Martin E.P. Seligman, Ph.D.
- American Psychiatric Association (1994). Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV). Washington, DC: American Psychiatric Association.
- SAVE - The Link Between Depression and Suicide
- Subsyndromal Depression, Encyclopedia of Human Behavior (Second Edition), 2012