P: Those who exercise regularly tend to have more consistent sleeping patterns than those who don't.
(A): Those who exercise regularly tend to have more consistent sleeping patterns than those who don't.
(A): (Those that tend to have consistent sleep patterns tend to sleep for longer.)
C: Those who exercise regularly tend to have restful sleep.
Neither assumption leads to the given conclusion. Since the first assumption matches the premise exactly, I'm guessing it is an error.
There are two holes that the correct assumption needs to fill in order to lead to the given conclusion. One is to equate consistent sleeping patterns with restful sleep. The other is to suggest that most people who exercise regularly achieve restful sleep, as distinguished from simple being more likely to achieve restful sleep than those who does not exercise regularly.
On the actual test, fulfilling the first of the above conditions would be enough to make an answer correct. It is actually pretty hard to make a one-sentence assumption that fills both holes.
Filling the first hole:
All (or most) people who have consistent sleeping patterns have restful sleep.
Filling the second:
60% of those who do not exercise regularly nonetheless display consistent sleeping patterns.
Filling both:
Most people have consistent sleeping patterns, which are sufficient for restful sleep.
