They can build gradually or strike hard and fast. Some of the symptoms may manifest as either too much or too little of the same thing. Mildly depressed people, for example, may gain weight as they seek comfort in favorite foods, and those suffering from more profound depression may lose weight as their appetites are deadened by sadness. Sleep patterns also may be affected this way. Some depressed people have difficulty falling or staying asleep, while others sleep more than usual but awaken feeling tired.
Depressed people tend to have lower levels of norepinephrine and serotonin.
If, for any reason, the amounts of these key neurotransmitters drop below critical levels, the result may be an endogenous depression that seems to come from nowhere, linger forever, sap our energy, and ruin our lives. We can't fully answer that question, although we know that genetics plays a major role. Depression, like other mood disorders, tends to run in families. A great deal of research has looked into possible environmental or psychological causes of depression. Some investigators believe that people who are pessimistic, often feel overwhelmed by life, or have low self-esteem are more likely to suffer from depression.