|Istanbul cruise photo by Jackson Gibbs|
Some of those 8-nighters (and longer sailings) will also include one or two nights on which the ship leaves the port at midnight. Like overnights, these late-night push-offs allow passengers ample time make long day trips outside of the immediate port area (say Cairo for Alexandria stops or Florence for Civitaveccia).
The one exception to the 8-day rule seems to be Caribbean cruises; no matter how long you sail on those, you're going to push off when the sun sets. Why? I'm guessing that the revenue in the casino on these "fun in the sun" journeys is just too important to the cruise lines' bottom lines to allow the passengers off the ship at night.