Often the first signs of bipolar disorder are severe moodiness, unhappiness, or other symptoms of depression. It is common for children with bipolar disorder to be diagnosed first with only depression and then later to be diagnosed with bipolar disorder after a cycle of mania or hypomania (a less severe form of mania). For more information on depression, see the topic Depression in Children and Teens.

A first manic or hypomanic episode can be triggered by a stressful situation or may occur without an obvious cause. It can also be started by certain medications used to treat other conditions. Drugs (such as antidepressants or stimulants) that are used to treat depression, attention deficit hyperactivity disorder (ADHD), and obsessive-compulsive disorder (OCD) are sometimes prescribed to children with bipolar disorder who have not yet been correctly diagnosed. These drugs can trigger sudden bouts of mania, sometimes with bizarre, aggressive, or psychotic behavior. (However, these medications are sometimes effective for children with bipolar disorder when they are combined with a mood-stabilizing drug.)⁴

In adults with bipolar disorder, mood swings usually occur over weeks or even months. In children, cycles usually occur more rapidly, sometimes within the same day (rapid, ultra rapid, or ultradian cycling). Frequently, children with bipolar disorder have difficulty getting going in the morning but then have intense energy later in the day. Often the mood shifts are continuous, rarely returning to a normal mood between extremes. Sometimes elements of depression and mania or hypomania may be present at the same time (a mixed state). These rapid and severe mood changes may make your child appear constantly irritable, and they can significantly interfere with your child's ability to function at school, at home, and with peers.

Children with mania can be more irritable and prone to temper tantrums or destructive outbursts than adults with mania. In a depressive episode, children may complain of headaches, muscle aches, stomachaches, or fatigue. They frequently miss school or talk about running away from home. They become socially isolated and overly sensitive to any kind of rejection or criticism.

While all teens may be rebellious or make bad choices from time to time, teenagers with bipolar disorder are more likely to show poor judgment, take risks such as breaking the law or having unprotected sex, and believe they are more powerful or important than they really are (delusions of grandeur) during manic episodes. A teen in a depressive episode may withdraw from social activities, do poorly in school, and have problems concentrating and sleeping.

Obsession with sex (hypersexuality) is common in children and teens who have bipolar disorder. Even young children may touch themselves, use sexual language, and approach others in a sexual way. Adolescents with bipolar disorder may be obsessed with sexuality and engage in risky sexual behavior. Hypersexual behavior is common in children who have been sexually abused; however, many children with bipolar disorder experience hypersexuality without having been molested.