Ok. If you want the distinctive or unique version, they experience srong periods of depression, where they are overcome with thoughts of suicide, and are often unable to function.
These periods of depression are followed by manic periods, in which the patient rarely sleeps, and focuses intently on something or someone. They become completely absorbed in their “project” to the exclusion of everything else, and without any restraint or inhibition. They become addicted to this period of what they consider productivity, or mania.
Then the cycle repeats itself again. Drugs such as lithium are effective for treatment and can even out the extremeness of the mood swings. They typically do not like the medicine, because it robs them of the manic periods. They will take the meds for a while and then go off of them. Then the cycle begins to repeat itself. Most of them will attempt suicide more than once in their lifetime. Many are successful. There is no cure, only treatment which is not always effective or well-tolerated.
There has been some scientific research into bipolar which I believe they have documented identifiable changes in brain function among those diagnosed with bi-polar disorder, so there is actually a diagnostic test for it. I will have to check on whether it is available beyond research. The symptoms are so severe, it is not usually necessary to run further tests.