Frenchman Michel Siffre Has Spent Months at a Time Living

Frenchman Michel Siffre has spent months at a time living in caves deep underground without the usual external markers of night and day.He found that without these markers, his sleep cycles tended to

A) stay exactly the same as his sleep cycle at home.
B) get longer with one stay settling into a 48-hour sleep-waking cycle.
C) get shorter with most stays settling into a 20-hour sleep-waking cycle.
D) become erratic with his sleep-waking cycles ranging from a short 20 hours to a longer 36-hour cycle.