What happens if people live in an environment in which the cycle of light and dark is other than 24 hours?

    *a.    They adjust better if the cycle is close to 24 (e.g. 25).
    b.    Within a few days they adjust to waking and sleeping on the new schedule, whatever it is.
    c.    They adjust better if the cycle is some multiple of 24 (e.g. 48).
    d.    They fail to adjust at all.