"Anatomically modern humans have lived on Earth for 200,000 years, and the creatures we descended from have lived on Earth for 541 million years. Stuff as dumb as the moon cycles affect us. How are we going to live somewhere that isn't exactly Earth?"

To further your (good) point about gravitational and diurnal differences, it is worth also considering the experience of native americans in the face of novel (to them) micro-organisms brought in by europeans.

These diseases were not from another solar system and they were related to animals that had local analogs ... and yet they were devastating (up to 90% mortality in some places) to the native population.

On another planet the bacteria, viruses and fungi (not to mention the potential discovery of previously unknown biological primitives) would be completely novel to us.

I wouldn't be shocked if there was 100% mortality outside of containment/quarantine after a week or two.

EDIT: On the other hand, imagine if we found that our genetic code did contain information related to what we found offworld ... wouldn't that be fascinating ...