Okay, a half-year later, I got this rebooting problem again. It started this morning after I turned off the sleep mode on my bed. I tried the same trick i used last time but didn't help. I contacted Samsung online chat, and they instruct me to do a complete reset by holding down the power button, I should then select "Recovery" option from the "Reboot Select Mode" menu, however, I don't see this menu, but only "continue" or "download". No matter which one I select, GearFit continues with rebooting again. At this point, the service rep gives up and told me I can either send it back to Samsung for service (takes about 2 weeks), or go to the store where I bought it (Best Buy) for help. So, I went back to Bust Buy, and they just gave me a new one.
By the way, I really love this little thing. I keep it with me around the clock. It goes with me to swimming pool, hot top spa, sauna, and shower without problem.