During the day, as the sun warms the Earth, rising air currents mix in the upper and lower atmosphere. This brings some of the high-speed wind energy from the upper atmosphere closer to the surface. (Because of friction with the Earth's surface, wind speed usually increases with height.) When the sun sets and the rising air currents stop, due to the lack of solar influx, wind at the surface diminishes.
The exception is when a front in in play during night hours.