Are you using a Flex or a One? Either way, you could try resetting your device from scratch, or else go into the Settings menu on the web site dashboard and try changing it from Standard to Sensitive, or vice versa. It could also be the device is getting "tapped" rapidly at night for some reason and turning itself off after you set it. I noticed if I shake my hands pretty hard (I wear the Flex) it will sometimes automatically activate the sleep mode (hence it could be shut off the same way, but I'm pretty sure you would notice unless you are sleepwalking).
