I think the recommended sleep is 7.5 hours for adults, but some people are OK with less so your 5-6 may be fine. It's hard to say, but if you're getting good quality sleep and feeling good in the mornings you may be one of those people.
I know someone who was getting 9+ hours a night, but still feeling exhausted in the mornings. During a sleep study it showed that they were rarely achieving the 'deep sleep' stage of the sleep cycle (where your body repairs, regrows tissues etc) or the REM stage, therefore the body wasn't getting a chance to works its magic.
Apparently the first REM stage usually happens 90 minutes after you fall asleep and lasts about 10 minutes. Then there are more 'cycles' throughout the night. It's really interesting stuff!