There are so many ways for living organism to face off the changes in light. That is not only for a human being, but also plant and animal are doing the same way. For example, crabs. The crab can move to the higher place when the night comes and come back to the lower place in a day. This term could help them to do every day and become the cycle in their live. Human also have a circle of their life, since the activities did every single day can it help them to do continuously the next day. People have a natural body clock that can manage them to going sleep or just to take a rest. Nonetheless, some people who productive in a job feels like confused because they don't have enough time to sleep. For people who live in the equator has equal time in a day and a night. Based on research, the inhabitant who lives without artificial light like a lamp will be easier to sleep twice in the day. They go to bed around 8:00 p.m. until midnight and then they sleep again from about 2:00 a.m. until morning. To sum up, people have a natural sleep cycle, thus, that is fair if they feel exhausted when they do not have enough time to do it.