Tonight I realized what this was.
Back in 1998, during my first (failed) attempt at college, I had a nasty little depressive episode. It was the sort where I was up all night because I couldn't sleep, and then asleep all day because I was up all night, sleeping for nearly twelve hours a day, feeling generally disoriented, and completely unable to recover myself from the nasty little emotional shock that had set it off.
I don't like to dwell on it. The past is always the past, but some of my past is an open book, and some is a closed book. That part of the past is not only closed, but locked as tightly as I can bear to keep it. There are some parts that were good, but the rest -- I describe it as "a black cloud" when I look back on the depressions. It's like walking through ice-fog in the dark, with no streetlamps to make it glow and provide illumination, just a darkness with occasional flashes of illumination. (I could probably have used this book then; I was certainly flailing about ever more wildly in my knowledge that I hurt enough to want to die but I didn't actually want to die die, just wanted the hurting to stop.)
I did have some emergency measures. When I knew I was on the edge of seriously falling apart, I had a temporary measure that would fix me up good as new and get me through the night unless something worse happened. I would take .75 liters of Jolt (I got it in the liter bottles from the little dorm store, and one time I wanted to know exactly how much it did take to get me out of the dangerous frame of mind) to artificially elevate my mood to the point where I could be made to laugh, and apply a comedy. Any comedy. It didn't matter which one, so long as it would make me laugh. Laughing would get me the rest of the way out of danger for the night, and I'd be decently all right. So I'd sit by myself in my room and watch a movie. Company would have been better, but bad company was more dangerous than no company at all.
150 mg of caffeine + 1 comedy = the ability to live until morning.
Needless to say, I don't ever want to go there again. And something small inside me still doesn't feel quite safe watching a movie by myself unless absolutely necessary to save what's left of sanity in order to save our life.