Overvolting: Just try and see if it works?

I have a couple of UV LED flashlights that aren't "high powered" and they work when I increased their voltage x2 or x3. One is a "CREE Ultrafire C3", whatever that means, because when I google that, many white lights pop up instead. The other one says, "TANK007 TK-566" and it has the AA extension tube for 2.4 Volts of eneloops.

I like how much brighter they are with 3.7V cells. Will I just shorten their lifespans by doing this, or do they even care? They both look like they're just a single 5mm LED, without a circuit board, but I'm really not sure, and wouldn't know how to tell.