In Can I significantly shorten the days on a planet that can support human life? I asked about changes needed to a planet to support human life with a much-shorter day (12 hours or so instead of 24ish), and learned that speeding up the rotation of an earth-like planet would be feasible. (Answers are divided on whether such a planet should have a moon.) In this question I want to ask about the physical and psychological effects on the humans living on such a planet.
Assume that these are human colonists from earth; they did not "grow up" on this planet but found it in their explorations and settled. They live in structures above ground and spend time outdoors, same as most of us. They're affected by the natural light and the (apparently?) moderated temperatures.
How would this shortened day affect humans over time? Would their circadian rhythm change drastically? Would there be psychological effects, or would it seem normal that you sleep "every other night, plus a bit"? How would their outdoor activities be affected by the shortened day?
On earth we can look at prolonged sunlight and prolonged darkness at high latitudes, but I'm not sure how to research this faster oscillation.