The length of time you sleep is not what matters, it's the amount of deep (REM) sleep you have that counts.
An average adult needs about five hours of REM sleep, and the rest of the time is taken up with going down to that level, and coming up again. So, if you have ten hours of sleep, you can spend 2.5 going into REM sleep, 5 hours in it, and 2.5 coming out again.
But - and this is where some people are lucky - you can go straight into REM sleep, then you only need five hours to be fully rested.
Famously, Lady Thatcher only needed five hours sleep, and that is the reason why.
But it's not something she, or anyone else who can do it, can boast about, you are simply made that way, it's not something you can learn.
So, if you feel OK on five hours, then that is how much you need.
Your body will let you know if you need more, you'll be sluggish and sleepy until you nap, or sleep for longer.
See how it goes, you'll soon get a feel for what is right for you.