It is not even an "idea". Is the consequence of nuclear power being so difficult to modulate to consumption. You give it for free by night, hoping that at least some people consumes, because the country is sleeping.

Solar energy aligns so much better, peaking its production when the consumption is high, going to zero when the consumption gets low, except for the diner time. In Spain we get really low prices (sometimes near 0) in the middle of the day, you don't have to plan or incentivize night consumption.