I'm going to be 38 this year and I don't understand how I still don't notice I'm manic until I'm in bed crying a couple of days later. 

I've noticed I start by deciding to exercise, which in turn helps me generate energy, which snowballs into extreme happiness/energy/productiveness (cue fast beating heart, adrenaline). 

Then something triggers me and boom. Serotonin levels depleted. Time to feel empty and sleepy for a few days