If it's safe for premature babies, it's safe for you! (more on that in a sec) 
First... some boring stuff for the nerds :
The Rejoover uses FAR infrared radiation - a natural form of energy that is present in sunlight and is part of the electromagnetic spectrum. No, it's not a scary futuristic technology. No, Elon didn't invent it. And no, it's not 'untested' - quite the opposite!
Infrared saunas have been around since the 1960s and millions have been sold around the world.
Fun fact: infrared heat has been proven to be so safe that many hospitals use it in infant incubators to keep premature babies warm after birth.
Another fun fact: some fancy yoga studios have begun putting infrared panels on their walls for extra heat because, apparently, those flexible freaks LOVE to sweat. So do we, we're just too stiff (read: lazy) to do downward dog.