It's been several years since I took my neurobiology class in undergrad. There has been a lot of research in this area. You're right that the clock does deviate

It's been several years since I took my neurobiology class in undergrad. There has been a lot of research in this area. You're right that the clock does deviate (I checked Wikipedia and it looks like in the absence of light humans' circadian rhythm cycles on just over 24hr). Exposure to light helps tweak the cycle.