as i learnt the sleep requirement depends on the age. when you are a child you sleep a lot initially 16-18 hours then it goes on decreasing with the age thats why you find old age people hardly sleep for 3-4 hours at a stretch .
secondly it depends on the activities that you do. generally,if you are physically active then you require more sleep then those not active.
thirdly the state of mind, people with depression and tension either increase or decrease the sleep hours depending on the body response.
lastly if this question is to save time without loosing health. then SHAVASANA right before the sleep is the right answer . though, initially it will increase your sleep hrs but as krishubpages123 said SHAVASANA immediately lets you into the deep sleep and also increases the hrs of deep sleep so the rest of the hrs are saved after practicing it for a few days.