Don't they loose a little ooomph at night and gain it back when the sun is beaming down on the cloud tops?
Sir James Lighthill in Fluid Mechanics of Tropical Storms, Theoret. Comput. Fluid Dynamics (1998) 10: 3-21 describes a hurricane as a heat engine with the warm seawater at temperature 300 degrees Kelvin as the heat source, and the stratosphere at a temperature of 200 degees Kelvin as the heat sink. By this model one would expect the intensity to increase at night as the stratosphere cools and the sea surface temperture remains essentially the same since Carnot heat engine output is the temperature difference divided by the warm temperature.