Brahmâ’s Night. A period of equal duration, during which Brahmâ. is said to be asleep. Upon awakening he recommences the process, and this goes on for an AGE of Brahmâ composed of alternate “Days”, and “Nights”, and lasting 100 years (of 2,160,000,000 years each). It requires fifteen figures to express the duration of such an age; after the expiration of which the Mahapralaya or the Great Dissolution sets in, and lasts in its turn for the same space of fifteen figures.
Source: H.P.Blavatsky - The Theosophical Glossary