I just don't see the point. Why not 3 8-hour shifts then? Is there any advantage to artificially shorten the perceived days? Other physically or mentally taxing jobs are also 8 hours, so I won't accept that as an excuse.
Nuclear submarine crews originally operated on 18 hour days so the sailors could adjust slightly within the 24 hour day.
They got six hours of sleep and twelve hours of waking time, in which they would engage in work and leisure. No commute is required, nor is there any need to talk with family or engage in recreation outside the workplace (submarine), thus lots of time spent traveling is removed.
It has since been changed to a 24 hour day where crews can operate on a watch schedule that evenly fits into 24 hours (2 12-hour watches, 3 8hr, 4 6hr), but research has not determined whether this improves sleep or work quality over the 18 hour model.
TL;DR: it’s perfectly realistic and has been common practice to have a nuclear submarine crew operating on 18 hour “days.”