Hi, welcome to the forum. From your history there is nothing wrong in your sleep pattern and it is absolutely normal. Before you use to sleep for long hours, you belonged to category of hypersomnia of central origin. In such condition person sleeps for a long time.
Eventually your duration of sleep came down to 6hrs. Even 6hrs of sleep is sufficient for you mind and body to rest. The whole day you are active and you don't have problem of day sleepiness, inactiveness, fatigue or other symptoms. Hence, you don’t have insomnia (sleep deprivation).
This is possible that there can change in the pattern of sleep depending on age, environmental factors, work environment, family etc.
If you experience any sleep deprivation problem consult physician. Take care and regards.
The first thing is to make sure you don't have obstructive sleep apnea. You can be fit, and not even snore to have obstructive sleep apnea. The fact that you're not dreaming and can sleep for long periods and still be tired is strongly suggestive. I'm willing to bet your performance is affected. You just can't yet. Consider seeing a sleep physician.