The circadian clock is biology's strategy for coping with the earth's rotation, the sun clock. What happens if the different clocks, circadian, sun and social are set to different times? What happens if we change our lifestyle so significantly that our clocks have trouble synchronizing to the 24-hour day? In his part of this lecture, Till will show you the consequences of living inside buildings away from natural light on one hand, and of exposing ourselves to artificial light after sundown on the other hand. He'll tell you about first insights into the consequences of living against the clock, be it due to social jetlag or shift work. He'll introduce you to different sleep patterns, some of which are thought to be healthier than others. And he'll explain to you why some, but not all, blind people are not synchronized to the 24-hour day. You'll need this knowledge to understand why our liver clock and our brain clock sometimes run out of sync with each other. And why living against our clock can make us smoke, drink, and put on weight. You'll understand why modern lifestyles have led to the new situation that changes sleep, a traditional output of the clock to its major input. The clock of some people only gets exposed to darkness when they sleep. [SOUND]