In experiments, participants averaged 12 or more hours of sleep a day for the first few days of unrestricted sleep. they then settled back to 7.5 to 9 hours of sleep a day. the unusually lengthy sleep time of the first few days suggests that participants began these experiments with
a) a sleep debt.
b) narcolepsy.
c) sleep apnea.
d) low melatonin levels.

a sleep debt.                        

Explanation:

Sleep debt is also known as a sleep deficit. It is the intensifying effect of not getting proper or enough sleep. A huge sleep debt can even lead to physical or mental fatigue. There are two types of sleep debt which include the results of total and partial sleep deprivation. According to a study, it takes almost four days to recover completely from the loss of one hour's sleep. Most adults sleep for only six hours each night.