Although you may put off going to sleep in order to squeeze more activities into your day, eventually your need for sleep becomes overwhelming and you are forced to get some sleep. This daily drive for sleep appears to be due, in part, to a compound known as adenosine. This natural chemical builds up in your blood as time awake increases. While you sleep, your body breaks down the adenosine. Thus, this molecule may be what your body uses to keep track of lost sleep and to trigger sleep when needed. An accumulation of adenosine and other factors might explain why, after several nights of less than optimal amounts of sleep, you build up a sleep debt that you must make up by sleeping longer than normal. Because of such built-in molecular feedback, you can't become accustomed to getting less sleep than your body needs. Eventually, a lack of sleep catches up with you.