Free-running sleep disorder in the extreme case of no sun and no concerted effort to entrain to sufficiently bright artificial light.
"Probably the best study to illustrate the effects of an unstructured life during the Antarctic winter is that of four Greenpeace volunteers over-wintering at Cape Evans Base 77°38′S in 1990 (Kennaway & Van Dorp, 1991). For the entire period of sundown (sun below the horizon for 126 d), all four (three men and one woman) showed free-running rhythms of all the variables studied: sleep, aMT6s, electrolytes, and cortisol. Each person free-ran with their own period initially, although two (a man and a woman) subsequently free-ran with the same period. With the return of the sun, all four resynchronized to the 24-h day."
"Probably the best study to illustrate the effects of an unstructured life during the Antarctic winter is that of four Greenpeace volunteers over-wintering at Cape Evans Base 77°38′S in 1990 (Kennaway & Van Dorp, 1991). For the entire period of sundown (sun below the horizon for 126 d), all four (three men and one woman) showed free-running rhythms of all the variables studied: sleep, aMT6s, electrolytes, and cortisol. Each person free-ran with their own period initially, although two (a man and a woman) subsequently free-ran with the same period. With the return of the sun, all four resynchronized to the 24-h day."
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3793275/