Isaac of Nineveh wrote in the seventh century:
“Once the Spirit comes to dwell in someone, the latter will not be able to stop praying, for the Spirit will never stop praying inside him. Thus, whether he sleeps or wakes, prayer will never be absent from the person’s soul. Whether she is eating or drinking, or sleeping or working, the sweet fragrance of prayer will effortlessly breathe in his heart. Henceforth he no longer prays at fixed times, but continuously.”
