I would like to think that the entire world could exist living in cities, so I become disappointed when I realize that there are downsides to urban life. Cities are certainly not "natural" places for us to live and as a result they can interfere with some of our natural functions. Life in cities can actually disrupt natural sleep patterns.
Major cities can be lit up almost 24 hours per day. When you walk around at night, you may not even have the perception that it is dark outside. Being constantly surrounded by light can interrupt the body's natural circadian rhythm. As a result, we wake up feeling tired almost everyday when it's time to go to work.
The human body naturally is set to follow guidelines of sleep from the sun. Before electricity, people would wake up earlier and go to bed earlier, not spending much waking time in the dark. But now as our lives are full of artificial light, sleep patterns have become much more irregular and sleep disorders have become more common. Most people who live in bright urban areas tend to wake up later and fall asleep later than people living further from urban centers. It's not exactly clear why this happens, but the consequences can be frustrating as people struggle to wake up on time in the morning and fall asleep at night.