The reason you might not feel rested, chickens, when you get upwards of 13h of sleep, might be due to the cyclical element of our sleep phases - I can't recall the specifics offhand, but imagine these stages as a series of hills; peaks/valleys - the peaks being closer to a more alert/wakeful state, and the valleys existing as deeper/less alert, more ''restorative,'' so to speak, sleep. These phases occur cyclically throughout the night, with more peaks/more shallow valleys the closer we get to waking up for the day [ordinarily within the 6-8h timeframe]. If you sleep about 4h, you might have gone through one phase of deeper sleep, & are creeping up towards a more wakeful state, which is why you might ''feel'' more rested/alert, than if you happen to wake later on & happen to wake while coasting back down into one of those valleys again.
..If that makes sense..
on that note.. Lex got 3h of sleep, & requires a nap.