Freerunning sleep can be sleep which is not adjusted, entrained, to the 24-hour cycle in nature. Sleep which is entrained to 24 hours but is not artificially regulated by alarm clocks etc. can also be called freerunning sleep.
Animals which are kept in a laboratory for research in constant conditions, that is in constant light or constant dark, will freerun. They live by their built-in circadian rhythms. Each day will be a little bit shorter or a little bit longer than 24 hours. It will be shorter if the animal's natural cycle is less than 24 hours long. The animals sleep and are active as usual, but their timing gets to be farther and farther off.
Scientists let the animals freerun and then do experiments to see what sort of signals are most effective at entraining them. They also experiment to see how long or short a cycle that animal can be entrained to. Some animals can be entrained to a 22-hour day, but they can not be entrained to a 20-hour day.
Sometimes people stay in a laboratory for a few days in very low light without any knowledge of the time. Such experiments show that adults, both older and younger, have a circadian cycle which is 24 hours and 11 minutes, on average.
Some people have a condition, a circadian rhythm sleep disorder, which makes them freerun all the time, even if they get out into daylight and eat regular meals. Many of these people are blind, but some of them can see. They have a long circadian cycle and they do not entrain. Scientists do not yet know why.
Some people believe they will be more healthy if they do not sleep on a regular schedule. They try to go to bed only when they are sleepy. They do not use alarm clocks. They call this freerunning, but since they are exposed to daylight and dark, it's not the same as freerunning as the scientists use the term.