Wow. That was really polished, addicting and fun, I loved it! I had a blast playing it and I wishlisted it on steam.
That said I am not quite sure about the way the health system is introduced. If I am correct, I think it mostly works like a timer (?) that decreases even more when you attack an enemy (?). It took me quite a while to realize that. I would personally like to know from the beginning that, apart from the enemies, "time" also damages you.
Also, when I finally understood how health worked, now the "Gain X hp every second" upgrade didn't make any sense especially combined with the "Kills Restore X health". Maybe in the future there'll be even more powerfull enemies and these upgrades will make sence, but as of now, at some point the whole health system became obsolete.
Take all this nitpicky feedback with a grain of salt, as it is just my personal opinion. I loved the experience overall and I would love to see where this game goes.
Wish you the best of luck!
Keep it up!