For many people, it is difficult to fall asleep at night. It feels like we naturally want to stay up later, even though we know we have to get up at particular hour in the morning to get to work on time.
This desire to stay up later is linked to the fact that humans operate on a 25-hour day. Studies have shown that humans, without access to day/night clues such as daylight or time pieces, naturally drift to a 25-hour day. In these studies, the patients would stay up one hour later each day, fitting into a 25-hour day cycle.
As humans we are designed to adjust, so we can manage a day that is one-hour off our natural cycle. We can easily adjust to going to sleep one hour earlier (24-hour day) or one hour later (26-hour day). When we try to modify our schedule by more than that, such as shift workers who adjust their schedule by 8 hours, then it becomes more difficult and side affects including health problems may occur.
This also reveals the importance of sticking more-or-less to a steady bed-time and wake-up time, even through our days off. Monday mornings can be very difficult when you start to drift your day/night cycle by staying up late on Friday, and even later on Saturday. Suddenly on Sunday you are wanting to adjust your day cycle by moving your bedtime up by 3-5 hours, which is a difficult adjustment. Therefore by Monday morning many find themselves groggy and sleep all day from sleep deprivation.