Usually, the air temperature in the sunboxes drops when the sun sets, and the pump turns off.
However, there is a time later in the evening when the heatpump is working harder (for evening heating or hot water), and the temperature of the glycol circuit in the ground loop drops, and then the sunboxes come back on again to retrieve more of the warmth left earlier... or to retrieve the milder night air-temperatures we are now getting.
This is the big benefit compared with using a pro quality flat plate panel. These are designed to deliver high temperature heat for boosting water to 50 degrees. As mine are only trying to warm the earth, they do not need short sharp high quality, they need much longer periods of low quality heat, ie warmth.... even in the middle of the night!