The simple answer is no.

BUT the water flowing over the American Falls and Canadian Horseshoe Falls is greatly reduced at night for power generation purposes.

During the summer once the sun goes down, the water flowing over the Falls is reduced from the daylight flow of 100,000 cubic feet per second to 50,000 - allowing hydro plants on both sides of the border to draw more water for hydro generation. Both the New York State Power Authority and Hydro One pump this extra water into gigantic reservoirs to be used at their call.

Once the cold weather sets in (November to April) even more water is diverted from going over the Falls. An additional 50,000 cubic feet per second is diverted  for power generation allowing only one-quarter of the water that could go over Niagara Falls to do so.  In the early 1950s when this was decided, authorities justified the diversion by saying there are fewer visitors to the Falls during this time.  However, today an argument could be made that this isn’t quite the case.

Albeit peak season is summer in Niagara Falls, the city is undoubtedly a year-round destination, drawing visitors throughout the frosty white winters to the hot, sunshine-filled summers! 

Of course if the Falls were completely shut off at night, that would mean you wouldn't get the opportunity to see the nightly Falls Illumination - a breathtaking sight!  Lit up every single night of the year, the illumination of the Falls occurs for a few hours beginning at dusk in varying colours.