Sleeping for only 6 hours at night can be okay for some people although you still might be feeling tired and need 8 or 10 hours of sleep instead.
For some people 6 hours of sleep is normal and enough while others may need to sleep 8 hours and if you cannot sleep longer than 6 hours but are still feeling tired in the morning then you might need to take some sleeping pills to help you sleep longer.
Or try drinking some bed time tea to help you relax and drift off to sleep.
Everyone's body and internal body clock is different though and trying to sleep for more than 6 hours per night could be a losing battle.
If you're feeling well rested and only able to sleep 6 hours at night then it's perfectly fine although if you're still feeling tired after 6 hours of sleep then you need more sleep.
Your bed might not be as comfortable as it should be either and you may actually be awake part time during the night instead of actually being asleep.
So you might need to try some relaxing music, avoid caffeine 2 hours before bed and try taking some sleep medication to help with insomnia.