text: sometimes i can sleep for 6 hrs and not be tired, other times i sleep for 15 and i'm still tired. why? i don't get it. last night i slept for 15 hours! and really didn't feel that refreshed. other nights i sometimes sleep for only like 6 and i feel fine. what's the deal? hello there you have gotten many answers and heres my two cents worth !  see for over 50 years i had sleep apnea and had your exact conditon but denied it!  last year i had a simple operation and was under anesthesia and simply stopped breathing for a minute or so ,,,  you know some doctors call it  death .  after they revived me and i escaped the hospital within the month i went to a sleep clinic for an evaluation!  my wife had been telling me for years that i had breathed funny but i ignored her ... just could not ignore  death!  anyway anyhow in one night they checked me as to how i did sleep.  what bothered my sleep.  the next session a few weeks later i was tested for the setting of the cpap machine the ideal pressure for it for me to have the ideal sleep.  we sleep in six levels of sleep one thru five our body is asleep but our brain only sleeps on level six or rem for rapid eye movement.  see it was like this for me. i would get into rem my body was on automatic but my auto breath would fail somehow ....  oxygen would get low the brain would drop out of rem and make the lungs gasp for air ... then would gradually go back to rem during the night at the lab this happened 20 times!  and i got to meet another disturber of mr rem it was my leg twitch which i have rls restless leg syndrome it disturbs the rem pattern of sleep also!  i used to have a high pressure job in shipbuilding and i would grit my teeth during my sleep  from work problems.. that disturbs rem i found out!  it is the quality of sleep not the quantity of sleep!  if you are not getting your rem your brain is not getting any sleep at all!  i do feel so much better than i have ever felt in my whole life! i always thought that sleep was sleep not so.  you might try one of those sleep labs for the night and they will tell you exactly very specifically how much you do need and what if anything is disturbing your sleep!  i am 61 and i can outwork a 20 year old but the cpap has taken at least 30 years off my age!  i love the darn thing!  it has improved everything!!   have a good day   aps