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.