If a person has certain moments when he feels happy, as if his life is complete and then suddenly becomes depressed for a short period of time, can it be bipolar disorder?

It can. In psych, that's usually how my professors describe it. You have to pay attention to the period of time though because the DSM criteria is a week or couple weeks if I'm not mistaken. The "manic" moments are often characterized by extreme happiness and energy, but the depression must include what they call "anhedonia" or lack of motivation/inability to experience pleasure. As you can see, it's basically severe high and low moods, but they don't alternate instantaneously. That's another disorder but I can't remember which right now. Bipolar disorder happens when mania and depression alternate in the person, for weeks usually. Hope this helps and thanks for reaching out!

There’s a difference between normal moodiness and bipolar. Everyone gets moody from time to time — puberty, pms, menopause, stress, exhaustion, too much exercise, change in environment — but bipolar moodiness is more extreme. People with bipolar can feel so happy at one time that they can feel so confident that certain dangerous risks can be taken such as drinking or buying way too many tickets at the lottery. It can also involve making all kinds of impulsive decisions in relationships, which can be big trouble for sufferers. When depression comes, it’s not just mild or even a little bit of intense sadness, it’s a really suicidal type of sadness. The type of sadness where someone would perform self harm or stop their daily routine completely at work or studying. It’s a very serious mental illness. If you are looking for more bipolar resources by professionals, Dr. Tracey Marks and Polar Warriors on Youtube can help.