Depression is very often part of a larger syndrome or condition of ill health. The effects of depression are both physical and emotional. The symptoms of clinical depression vary from patient to patient, depending on their brain chemistry and the underlying causes of their depression. Symptoms can be sustained for weeks to years, and may include:

Fatigue and decreased energy
Difficulties making decisions, focusing, or concentrating
Persistent sad, anxious or "empty" feelings
Feelings of guilt, worthlessness, or helplessness
Feelings of hopelessness or pessimism
Irritability
Restlessness
Loss of joy for things they used to find pleasure in
Overeating or appetite loss
Thoughts of suicide or suicide attempts
Chronic headaches, pain, or digestive problems that do not resolve
Disturbed sleeping patterns, including insomnia, oversleeping, or waking early and being unable to sleep again

The causes of depression remain unclear, although it is thought to be brought on by a number of social, psychological, and biological factors. Genetic variations, traumatic events, or severe emotional distress may put a person at risk for developing depression. Depression can also be triggered by low self-esteem, stress, medical illness, or drastic changes in life pattern.