The Nissin MF18 Macro Ring Flash will go to sleep after 10 seconds of inactivity to conserve battery power, and it will fully power-off after 10-60 minutes of inactivity (depending on the flash's "Auto Off" setting).
You cannot disable the sleep mode on the MF18, but if the flash goes into sleep mode due to inactivity it will "wake-up" when you press the camera shutter button, or any of the buttons on the back of the flash's control unit.
If you were using the modeling light when the flash went to sleep it will turn back on when the flash wakes up.