Insomnia, early-morning awakening, and oversleeping are all common. Thoughts of death or suicide are not uncommon in those suffering from severe depression. Restlessness and irritability among those who have depression is common. A person who is depressed also has difficulty concentrating, remembering, and trouble making decisions. Feeling down or feeling like you've got the blues is pretty common in today's hectic, fast-paced society.
People are more stressed than ever, working longer hours than ever, for less pay than ever.
That's completely normal. What differentiates occasionally feeling down for a few days from depression is the severity of the symptoms listed above, and how long you've had them. Typically, for most depressive disorders, you need to have felt some of those symptoms for longer than 2 weeks. They also need to cause you a fair amount of distress in your life, and interfere with your ability to carry on your normal daily routine. Depression is a severe disorder, and one which can often go undetected in some people's lives because it can be insidious.