Endogenous pacemakers (EP), or your internal biological clocks, can be altered by exogenous zietgebers (EZ).
An example would be the suprachiasmatic nucleus, the main EP located in the hypothalamus, sending signals to the pineal gland to release melatonin (a hormone that makes you sleep) when it receives signals about the levels of light in the environment from the optic nerve. In this case, these two (EP and EZ) are responsible for the time you sleep and wake up. This is of course assuming that you aren't dead tired and your body doesn't badly need rest. Those people in the arctic are also affected by this albeit with factors other than light playing key roles. This would also be the reason why you experience jetlag.
Another example would be the gonadotropin-releasing hormone (GnRH) neurons, also located in the hypothalamus, that stimulates the pituitary gland to produce follicle stimulating hormone (FSH) which is responsible for egg development causing the estrogen to rise in women, being affected by the pheromones secreted by other women. In this case, the EP and EZ are responsible for the menstrual cycle being shortened or lengthened. So if you're a girl and you smell another girl's armpit, keep in mind that she may be affecting your menstrual cycle, although for what reason you would be doing this I have no idea.