Your body invented the clock long before the Swiss got wise to the idea. It figured out that the planet revolves around the sun in a 24 hour cycle. It then adapted itself to feel awake when the sun arose and feel sleepy after the sun set. Its called your circadian rhythm. Widely observed in plant, animals and fungi, it follows a cycle roughly 24 hours everyday. It uses external stimuli such as light and dark as triggers. These external triggers are called zeitgebers (German for time giver). The clock though is internal. Call it your very own Omega.
These rhythms evolved to cope with your natural environment. This helped prepare you to meet the day. Or give your brain a rest so that it could refresh. The cycle coincides with other biological cues such as lower and higher body temperature, brain waves and brain activity, thirst, appetite (but for those people who are always raiding the refrigerator at 2 a.m.) and cell regeneration. Strange phenomenon have been observed in part of our world that do not follow the regular sunrise / sunset cycle and live in 24 hour light for example. Plants and animals there are unable to follow a regular circadian rhythm.
Human beings who do not live a lifestyle broadly in consonance with this rhythm show signs of fatigue and stress. Pilots who cross several time zones in a day regularly for example. This extends to also increasing the risk of obesity and diabetes. In short, this means listen to your grandmother, stop burning the might night oil, kicking up a storm in a discotheque, or spending sleepless nights watching Game of Thrones. Your body does not appreciate it. Strangely, being exposed to certain types of light can also upset your body in what is called social jet lag. Count sheep if you cant sleep but do sleep on time.