I think another compelling reason which make sleep is so important is the compound called melatonin which act as an anti-oxidant. Melatonin is secreted by the pineal gland, and its role is to induce sleep and when an individual has fallen asleep, they will remove oxidant particularly reactive oxidant species (ROS). ROS can cause cellular damage. Timing also is as equally as important as the sleep itself. Melatonin are usually high between 10-12 am. That's why sometimes it is difficult to fall asleep after midnight; the melatonin starting to decline. If we want keep our cell intact and stay young, pay attention to the time we sleep.