Your hot tub will not freeze overnight unless the water stops circulating and you live in the Arctic. If your hot tub loses power, and assuming there is a cover and reasonable insulation, you will have at least a few days before water starts to freeze.
Hot tubs most at risk of having problems with freezing are those with external drains. The drain will actually freeze very quickly, break, and then let the hot water run out, taking with it all of the heat energy that was keeping the rest of the spa from freezing. In that case you can lose a hot tub in a matter of three days. That is the number one reason Royal Spa does not put external drains on the outside of their spa cabinets.
Royal Spa hot tubs are designed to maintain temperature for a very long period of time. Our interior cabinets are sealed and insulated and do not have voids or open venting areas for heat to escape.
Most people keep their hot tub at 102° to 104°. Starting at that temperature you would have approximately 7-10 days if the spa lost power. If you have a power outage, keep the cover on and open the cabinet door only when you have to.
For those of you who are science geeks here’s an article about when water MUST freeze: https://phys.org/news/2011-11-supercool-doesnt-.html