The ego in his turn puts down part of himself for the same purpose into lower worlds, and we call that part a personality, because the Latin word _persona_ means a mask, and this personality is the mask which the ego puts upon himself when he manifests in worlds lower than his own.

Just as the ego is a small part and an imperfect expression of the Monad, so is the personality a small part and an imperfect expression of the ego; so that what we usually think of as the man is only in truth a fragment of a fragment.
The personality wears three bodies or vehicles, the mental, the astral and the physical.

While the man is what we call alive and awake on the physical earth he is limited by his physical body, for he uses the astral and mental bodies only as bridges to connect himself with his lowest vehicle.

One of the limitations of the physical body is that it quickly becomes fatigued and needs periodical rest.

Each night the man leaves it to sleep, and withdraws into his astral vehicle, which does not become fatigued, and therefore needs no sleep.