How would the human body adjust to sleep times if we were to live in a place with different day lengths?
Humans have evolved for 24 hour days and our bodies would not adapt well to this short of sleep/wake cycles (whether or not they were born there, unless they have been there for many generations and have been able to evolve for the new time). Our bodies would still want to spend about the same amount of time sleeping and being awake.

If we tried to adjust our sleep cycles to this planet with 8.5 hour days, we would experience many of the stresses that occur during jet lag (Google Scholar search), which can cause depression. Jet lag is identical to experiencing one day which is shorter or longer than the typical 24 hours.

The sunlight helps set our circadian rhythm, but 8.5 hours would be too fast. I thought I heard for an experiment that used mice that supports this, but I still haven't found the reference yet.

Nbogard referenced the article Plasticity of the Intrinsic Period of the Human Circadian Timing System, which showed that the circadian rhythm in humans can be altered by up to 0.65 hours by the use of lights. This is consistent with what I mentioned that the sunlight helps to set our circadian rhythm.

The best way to adapt would be to use artificial lighting to keep your days about 24 hour days (artificial light during waking hours, heavy curtains during sleeping hours), as mentioned in ThePopMachine's answer.