Ok. Now we know you are running a current edition of ubuntu (2015) ON A 60 gig hard drive with a 1 gig /swap partition
and to teach you how to read the free command you posted. I give you this
Sleep and things like that are related to ram and how much is used and how big a /swap partition is because the operating system loads into ram plus /swap to restart. Newer Ubuntu uses more ram to operate because of Unity desktop or what
ever desktop you are using. So more ram space is required to bring it out of sleep.
1 gig of /swap is pretty small. But Free shows 993392 total which is more ram than I posted in my free readout and my Dell has 5 gig of ram. Not owning Mac gear and being hip on how they operate.
I am afraid I am not going to be much help here. You look to have enough ram to do what you want. /swap is kinda skimpy but
10 gig of physical ram sticks should be plenty for sleep to work.
So sorry, Some Ubuntu Mac Mini user will have to pick up the ball and run with it from here.
Good Luck, Rok