If he should wake up during the night suddenly, with thoughts reverting to spiritual things, it is a good time to meditate upon them. It is not necessary to get up and dress, nor even to assume a sitting posture. He may even feel a kind of internal shock which precipitates him out of sleep into wakefulness in the middle of the night, after which he will find it difficult to fall asleep again. This, too, is a signal to start meditation immediately.
-- Notebooks Category 4: Elementary Meditation > Chapter 2: Place and Condition > # 53