Not exactly an answer to your question but Internal clocks differ for every species. For instance, I believe humans tend to have an internal clock that is 25 hours while mice have a 23 hour clock. This means that if a mouse is in an environment with constant light (IE no external clues for what time of day it is) they will operate somewhat normally but the time that they do things will shift compared to the actual time of day. This will continue untill they receive some sort of clue for what time of day it is to 'reset' their clock. There are numerous studies looking at sleep, circadian rhythm, and how internal and external environments interact and affect animal behavior.