A person who is straight when awake, but in dreams have sexual fantasies about others who are the same gender as him/herself. usually happens when stressed, having doubts about a relationship or after watching a chick flick/action movie.
EXAMPLE:

Sarah: Hi john.

Sarah's BF John: hey sarah have any cool dreams lately?

Sarahs thoughts: dont say anything about the dream you had last night about mary....

Sarah: nope

John: oh okay.

Sarah's thoughts: i knew i shouldn't have watched "jennifers body" last night... damn megan fox turning me into a Diurnal Bisexual..
by asian_waffles February 10, 2010
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