On planet earth, the day is roughly 24 hours long.

On the ISS, a day is about 90 minutes long, but they fake 24 hour in
terms of sleep/awake cycles.

Have they (or do they plan) on experimenting with whether the human body
could adapt to a different length of a day ? (for instance, a 20 hour
day, or 30 hour day etc etc).

mars appears to have similar length of a day compared to earth, so I
guess such experiements may not be so important afterall, but still
would be interesting to know if the body can adapt to a change in day
duration and what proportion of sleep/awake is needed.