Very little sleep and sit upright

Buddhist monks or yogis sometimes participate in long retreats with very little sleep and sit upright in a meditation
posture for 24 hours a day for considerable periods of time. In that situation, the sleep is very light and one never
goes very deeply into unconscious states. Obviously these practices are very intense and not particularly designed for lay people like you and I. They may not really mesh with living fully in the "world" as we do.

from: A Buddhist Sleep Lesson: Awakening From The Day Dream; by: David Nichtern,Senior Shambhala Buddhist teacher