I know that BL had been to the doctor about "blackouts", where he fainted for no apparent reason. I also know that very very fit people with extremely low blood pressure can get woozy or faint from getting up too fast. I don't know if this was the reason, but something like that combined with very high stress levels (remember BL was comparitively poor prior to his Hong Kong movie successes) may have had something to do with it.
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"In case you ever wondered what it's like to be knocked out, it's like waking up from a nightmare only to discover it wasn't a dream." -Forrest Griffin