The suprachiasmatic nucleus of the is responsible for regulating your circadian rhythms. In this way it plays a very important role in your sleep-wake cycle.

tiny region of the brain in the hypothalamus, situated directly above the optic chiasm.

the answer is true when observing or determining population size, a direct observation is use, in which each organism is counted in a population or in an area. one benefit of this measure is that you get the exact population count but it will take a long time to  determine  the  size  of a  population.