The depression in bipolar disorder is not necessarily the mild type, especially in adults. It may appear like a clinical form of the disorder such as dysthymia or seasonal affective disorder; or it could appear more severely as a form of major depression. When you are depressed in bipolar type of disorder, you tend to experience painful sadness, negative thinking, and indifference to things that used to bring you happiness.
You feel woebegone but cannot bring yourself to do anything about it; and you feel drained and tired, but you cannot do anything about it because you still can't get enough sleep. Adults who suffer from bipolar disorder have been known to seriously consider suicide when they are in the depressed phase of the cycle.
In the manic phase of bipolar disorder, you are overexcited without reason. You have all of these creative and inspirational thoughts at the same time but you cannot focus on any of them long enough to make them happen. You find yourself irritable because no one understands your rapid-fire vocalizations, and they don't seem to want to go along with your self-destructive behavior. Of course you aren't getting a lot of sleep either, and nothing can stop you from making indecent sexual advances, buying extravagant gifts, gambling and wasting money, and generally getting nowhere.
It does not take a lot to observe either of these symptoms in an adult. However, being able to notice both sets of symptoms to make the correct diagnosis often takes careful observation of the patient so that you don't end up with a misdiagnosis and treating them for the wrong thing. Sometimes the cycles are years apart and sometimes they can cycle several times within a single week. This is called rapid-cycling bipolar disorder. Surely you can't identify these on your own, without the right professional help.