It is only in animal and man that we find the astral forces working in the form of separate astral bodies. This accounts for their capacity for sensation and volition. Besides the alternation of birth and death, they experience the rhythm of sleeping and waking. Sleep occurs when the astral body leaves the physical and etheric bodies in order to expand into its planetary mother-sphere, whence it gathers new energy. During this time its action on the physical-etheric aggregate remaining upon earth is similar to that of the astral cosmos upon the plant.