One of the common arguments against sleeping without alarm clock is that man tends to oversleep if allowed. (In the way people usually overeat when have enough food.)
In reality, people do not oversleep because we do not have a mechanism to accumulate sleep. For food, we have fat to accumulate energy, but there is no reservoir for sleep. With that respect, it is more appropriate to compare sleep with water (for we don’t have reservoir for water either).
One of the “evidences” for oversleeping is that people usually sleep much-much more when they allow it (like 12 or even more hours). The thing is, the organism is trying to compensate for sleep deprivation. Once you compensate, you won’t sleep to much. e.g., I usually settle at 8-9h