After the Heat is "turned off" and the Wind "leaves", the CropCircle is still literally "hot" due to the microwave/MASER energy. I believe the complete cooling process lasts from minutes to an hour or more. In the "Overton Oval" Formation (England '93), the "top" Circle was transmitted the day after the main Formation arrived. Two phases of the "settling" of the stalks within the Circle's new addition were witnessed by several researchers in the early morning hours. Also, reports from earlier years have described instances of researchers coming upon "still-twitching" crop - i.e., crop which was in the throes of the cooling/settling/rehardening process.
I think that this is an important part of the answer as to why the Circles arrive during the hours of darkness: The Heat must have time to dissipate so that, come dawn's light, any visitors eager to explore a newly arrived Formation might do so in safety. The CircleMakers demonstrate that they are as well aware of our physical limitations as they are of our topography, geology, Gaian Energies - and the innermost thoughts we send them during meditations.
Though the Heat and Wind Energies do their work then disperse, the SoundBarrier remains in place for a "long" time. My work with the Chehalis Formations (which arrived in July '94) in Washington State confirms that the SoundBarriers of both Pictograms are still dowsable fully eighteen months after their arrival. The Energies remain even though the original wheat crop was harvested, the soil tilled, then the field was replanted, tilled, and harvested twice again (with oats in 1995, then wheat in 1996) - yet the SoundBarrier remains, and the Patterns can be clearly dowsed and delineated today!
In order for the SoundBarrier to remain dowsable so long after its Transmission, it might seem that on-going Energy infusions would be required to recharge or maintain it. As startling as the concept may be, I believe that the SoundBarrier might well be receiving additional Energies. While I don't consider this Sound to be "alive" in the accepted sense, I have wondered on many occasions if it might not indeed be "aware" at some level, imbued with a type of consciousness. Certainly there is a Presence within the Formations.
Beautiful Galactic 'S'-swirls: left, in galaxy M51; right, in centre of Telegraph Hill Formation