Adrenaline When you sad or tired you cant to do something, but if happens something horrible or great - you can find a forces which make you a stronger, for a bit time) And it is really helping) What makes your iris of eyes wider, and giving to you euphoric feelings?))) Yes it is) Your soul want to fly and your body becomes like a machine. And you wanted to feel it again))) Yeah... As for me) I can feel it when I listening euphoric or rocky music, when I'm running as fast as I can, and I want to run run run!! Fresh wind and bright light can make me run faster. Flame would burst in my heart))) But sometimes, in usual reality, where nothings interesting and where air smells like a sadness - we can remember our wishes, or something good - and we can understand that sadness is for a bit time too. And our eyes again becomes more seeingful, and your soul can to come out from four gray walls to fresh wind and bright light.