That is correct. The 6 hour on-watch period is based on maintaining full awareness. With a 3 watch-section crew and "18hr days," you'll have round the clock coverage for all watch stations. Like you said, we'd wake up, eat, go on watch for 6hrs, get relieved by the next watch section, eat, do maintenance/training/etc. for the next 6hrs, and then sleep for the final 3rd of our "day."