Nammu, I've actually read the opposite. That people with bipolar disorder tend to experience hypersomnia in depression more than insomnia. I also once read that you need not fit any tendency and that if one tends towards insomnia in depression that it doesn't mean you're not bipolar.

See Bipolar and Excessive Sleep Solutions | HealthyPlace.

" In major depression, hypersomnia is common, with about 30% of people experiencing it. However, in bipolar disorder excessive sleep is even more common. Across studies, 38–78% of people with bipolar disorder experience hypersomnia and it is highly recurrent."

I believe it comes down to what Dr. James Phelps proposes in his "Wave Theory". See the chart in his article at Rapid Cycling And Mixed States As Waves – Psych My guess why people with bipolar may tend towards hypersomnia in depression is because of some form of crash after hypo/mania and/or the polar opposite effect to mania, in terms of neurotransmitter goings on.