Sleep is of course not a waste of time but an essential behavior. Sleep is important in many aspects to the human body. It is when the things we come across and learn in day to day life are organized and stored. This is said to be formation of new connections (known as synapses) between neurons in the brain. So, depreviation of sleep leads to poor memory. That is the reason why neonates and infants tend to sleep majority of the time of the day as they learn a huge load of information in a very short period of time. (This is especially important for students - primary, college or university - to sleep for at least 4-6 hours a day in order to remember the facts they learn more efficiently).
Additionally sleep is the time when growth occurs and the biological processes have a rest and recover from minor damages which had occurred during the day time when the body is active. Hormones, like growth hormone which is essential for growth, are secreted mainly during sleep. Also the immunity tend to drop with sleep deprivation.
So, in terms of medicine and human biology sleeping for at least 4-6 hours is a must!!