Phap Hien was a Vietnamese Zen master. He lived in the late 5th century and early the 6th century. When he was a young monk at the Phap Van Temple he met Vinitaruci (?-594) who was one of the successors of Seng-tsan, the Third Patriarch...
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You can sit at some time in the early morning if your working schedule allows you to do so. In the early morning it is often less noisy than other times during the day and the atmosphere is still more pure than later in the day or in the evening.
You may sit in the evening about 9:00 P.M. or later. It would be better if you start to sit about two hours after your meal because if your stomach is still full it will be not good for sitting. What has just been said is for a beginner, and if your are an enthusiast practitioner, whatever time would be good for you.