If you can keep the temperature within 5-10°C of your day time temperatures, that is ideal. So, if you're hitting 27°C in the day (for example), you should aim to go no lower than 17°C at night.
In general, I'd aim to go no lower than 15°C at night, but I've gone as low as 12°C and had no issues - sometimes you can't help it! I'm sure many have gone lower without significant problems.