If someone has the following symptoms maybe they have depression as an illness and they do not simply feel sad or low:

• Pervasive low mood and tearfulness, unresponsive to and out of keeping with current circumstances.
• Other mood tensions including tension, irritability and anxiety.
• Insomnia (typically waking early)
• Reduction in appetite, weight loss and constipation
• Lack of energy and tiredness/lack of stamina
• Loss of libido
• Loss of concentration/forgetfulness
• Feelings of low self-esteem, guilt and helplessness
• Thoughts of suicide

A depressive episode is diagnosed if 5 or more of these symptoms last most of the day, nearly every day, for a period of 2 weeks or longer.