Ive had some mild bouts about 10 years ago..
Wake up in the morning(perfectly fine night before),open eyes in morning and a dark cloud behind eyes..Had 3/4 days off work and as self employed person 3/4 days with no pay and pissed off customers!! Recurred once every few months but better managed after previous experiences.. No idea why after racking my brain so just put it down to some kind of brain chemical imbalance or simply life in the UK..
Used to wake up in the morning a little nervous to open eyes as the cloud was either there or it was simply gone!!
Sigh of relief if it had gone and I was suddenly a different person..
Previous to this experience I would not take the subject seriously..No problems for 7 years whatsoever since and I can only describe my problems as mild compared to how others have described what they have experienced..
"Poor buggers" is my only comment apart from think your self lucky if you have managed to avoid the black dog...In whatever form.