When you want to complete all your tasks in one night so the next day you can continue with the next task or full speed to complete your job on hand so that the next day you can free the whole day and enjoy your planned activities. You may end up either sleep the first half of your day or you continue your plans with tired physical and mind. Sleeping late results to waking up late because body recovers from the energy you used in the evening. We each have our own personal rhythm and specific needs for sleep but generally you need to sleep for about 7-8 hours so that your body will not become weaker and feel tired during the day.