As bio-engineered artificial units, the grey were not affected by the cycles of energy on our world. They had been designed to function reliably for a long period of time, and continued to do so even after the Anu were gone.
The grey were advanced technology, and given the Anu’s distrust of humanity, they were carefully programmed to only take orders from the Anu themselves. However, it was normal for the Anu to be in hibernation for thousands of years when the energies were strong.
In their absence, the grey performed routine maintenance tasks on Anu structures both above ground and below and carried out surveys in a variety of areas that were of interest to the Anu.