I have pretty severe mood swings myself, as does my partner and many of my friends. Depending on the severity of your symptoms, there comes a point where medication really is the only thing that will pull you out of a cycle, but there are always things you can do to at least mitigate it.
Vitamin D isn't the cure-all it's sometimes touted as, but I have noticed that when I take some every day, it really helps keep me from just randomly falling into a depressive spike, and while I don't exactly have data to back this up, I feel like I recover from them a little faster, too.
Exercise will help with both depression and sleep patterns. I know, trust me, I KNOW, everybody says that and it seems so hokey and stupid and, like, missing the point. But it really, really, really does help. Even just getting out of the house and going for a walk will do wonders, especially if you set out with a goal in mind--getting a candy bar and soda from the gas station, or walking just long enough to listen to a podcast or an album on your headphones. The trick is, you have to keep it consistent--no walking three miles one day, and then not walking for a month.
Depending on your tolerances and tastes, caffeine can help a little, in that it will keep you from feeling "foggy" and unable to concentrate or motivate yourself. I'm a coffee nerd, so I'll go through a French Press every morning, but even just a cup of tea when you're feeling down can help.
Okay, so this is all about prevention--you asked about maintaining in the middle of a crash. Unfortunately, in my experience, once you're in a crash you kinda just have to ride it out--but, there are things you can do here, too. Getting off the computer really does help--I know, see above re: exercise, but the trouble with the internet, especially when you're feeling down, is it's WAY too easy to find something that will just upset you more, and the "reload page" reflex just perpetuates the cycle in your own head. I think that's much more the issue than the blue light thing (although I use f.lux, too, so take from that what you will!). I guess I would ask, why is getting off the computer after, say, ten o'clock a hard thing for you to do (they said, writing at 2:15 am), and does it balance out the problems with your sleep patterns?