Depression.

If you have some of these symptoms, it doesn't necessarily mean you are depressed! We all get fed up at times.
Constant feelings of sadness, irritability, or tension
Decreased interest or pleasure in usual activities or hobbies
Loss of energy, feeling tired despite lack of activity
Change in appetite, with significant weight loss or weight gain
Change in sleeping patterns, such as difficulty sleeping, early morning awakening, or sleeping too much
Restlessness or feeling slowed down
Decreased ability to make decisions or concentrate
Feelings of worthlessness, hopelessness, or guilt
Thoughts of suicide or death
If you are experiencing any or several of these symptoms, you should talk to your doctor about whether you are suffering from depression.
If you are in an immediate serious crisis please contact your doctor or go to your local hospital or A & E.
Dysthymia is another mood disorder. People who have it may feel mildly depressed on most days over a period of at least two years. They have many symptoms resembling major depression, but with less severity.
Symptoms of depression may surface with other mood disorders. They include seasonal major depression (also known as seasonal affective disorder), postpartum depression and bipolar disorder.
Seasonal Affective Disorder has symptoms that are seen with any major depressive episode. It is the recurrence of the symptoms during certain seasons that is the hallmark of this type of depression.
Postpartum Depression is a type of depression that can occur in women who have recently given birth. It typically occurs in the first few months after delivery, but can happen within the first year after giving birth. The symptoms are those seen with any major depressive episode. Often, postpartum depression interferes with the mother's ability to bond with her newborn. It is very important to seek help if you are experiencing postpartum depression. Postpartum depression is different from the "Baby Blues", which tend to occur the first few days after delivery and resolve spontaneously.
Bipolar disorder, another mood disorder, is different than major depressive disorder and has different treatments. For more information go to bipolar.com.

Whatever its cause, depression is not just a state of mind. It is related to physical changes in the brain, and connected to an imbalance of a type of chemical that carries signals in your brain and nerves. These chemicals are called neurotransmitters.Some of the more common factors involved in depression are:
Family history.
Genetics play an important part in depression. It can run in families for generations.
Trauma and stress.
Things like financial problems, the breakup of a relationship, or the death of a loved one can bring on depression. You can become depressed after changes in your life, like starting a new job, graduating from school, or getting married.
Pessimistic personality.
People who have low self-esteem and a negative outlook are at higher risk of becoming depressed. These traits may actually be caused by low-level depression (called dysthymia).
Physical conditions.
Serious medical conditions like heart disease, cancer, and HIV can contribute to depression, partly because of the physical weakness and stress they bring on. Depression can make medical conditions worse, since it weakens the immune system and can make pain harder to bear. In some cases, depression can be caused by medications used to treat medical conditions.

Other psychological disorders. Anxiety disorders, eating disorders, schizophrenia, and (especially) substance abuse often appear along with depression.