This is just like turning back the thermostat in your house at night.
The amount of energy lost as the water cools slightly will have to be
put back again in the morning, when it is reheated, so that part is a
zero sum game. What you save is the heat loss through the tank being
slightly less at the lower average temp while the heater is off. If
the tank is well insulated, as it should be, then I doubt you will save
enough to make it worthwhile.

In some areas, power companies offer hot water heaters on seperate
meters that only provide power during off peak hours. The rate is
lower for the consumer and it helps reducte the power company peak load
during daytime. As suggested, I'd make sure the pipes are insulated,
hot water pipes as far as practical and the cold water pipe too for a
few feet from the water heater. You could also put a heat trap in the
hot water line, which is basicly an upside down U, close to the tank,
so that hot water will not convect through the pipe.