If a sad feeling goes on for too long, hurts too deeply, and makes it hard for you to enjoy the good things about your life, it's called depression.

Here are some of the signs and symptoms of depression:

• feeling empty or numb
• feeling hopeless (like there's nothing to look forward to)
• feeling guilty or worthless
• feeling lonely or unloved
• feeling irritable and annoyed a lot (every little thing gets on your nerves)
• feeling like things are not fun anymore
• having trouble keeping your mind on schoolwork or homework or getting bad grades
• having trouble keeping your mind on things like reading or watching TV or not remembering what a book or a TV show was about
• having less energy and feeling tired all the time
• sleeping too much or not enough
• not eating enough and weight loss or eating too much and weight gain
• thinking about death or thinking about suicide
• spending less time with friends and more time alone
• crying a lot, often for no reason
• feeling restless (being unable to sit still or relax)
• having certain body feelings, like lots of stomachaches, headaches, or chest pain

People who have depression may not know it. Often it's a parent or teacher who notices behavior changes like the ones in the list above. Depression can run in families. Having a parent who gets depressed makes it more likely for a kid to become depressed.

When children have depression it is usually coming from a long period of sadness after the loss of someone really close, such as a parent; long-lasting problems at home, including violence, illness, divorce, or alcohol or drug use; child abuse or neglect; rape; and long-term illness, burns, or accidents. But sometimes kids may be depressed for no apparent reason.

link:http://www.kidshealth.org/kid/feeling/thought/sadness.html