Because it’s all of them it’s most likely an issue with your zigbee network connection to the bulbs. As Brian mentioned, there is a known issue with the GE links disconnecting themselves, but I’ve never seen or heard it happening so fast, consistent and widespread.
18 bulbs myself, with rare issues. Maybe one bulb a month needs a reset.
It could be interference with you or your neighbor’s wifi, so it might be good to check channels. Also take a look at this thread/post about power company meters that operate on zigbee and cause problems.