It’s simple... In 24 hours we spend 8 hours sleeping so in a day we spend 1/3 rd of our time sleeping. So by corollary we understand that 1/3 rd of the life is spent in sleeping.
On the other note and a very different one. We all live in a life of future... by this I mean we always live in a world of I will do this; i will do that and what I need to do for the future. Even great philosophers have said the same thing. Dream, dream big... I do not understand why we need to live in the future. If we strive not to live in the past we need not to strive to live in the future too. May be it’s just a thought. Evolution may mean living in the future, but isn’t it taking our precious present away from us.