Getting enough sleep is one thing. Consistently sleeping until you are not tired is another thing entirely.
Everyone knows that if you exercise well, you tend to sleep well. Unfortunately exercise can be overdone. If you regularly begin your practice at night after you are already tired you run the risk of giving yourself insomnia; tapping into that deadly "second wind," and depleting your yuan qi.
Each internal organ has two basic ways of moving, the generative and the re-generative. For example the liver uses jelly-fish-like movement to distribute blood out to the muscles when it is in the active generative cycle. When you get a good night's sleep blood draws towards the liver and it swells up. During the re-generative cycle the muscles need less blood.
In addition to the quality of its movement, the shape or tone of an organ is indicative of its functioning.
The first level of internal martial arts is usually try to do nothing internal so that you can just relax. The second level involves feeling the internal organs moving but do nothing with them. At the third level, we connect the movement of the organs to the movement of the limbs, head and torso. All three levels are actually infinite; we never stop practicing them.
All three of these practices tend to involve changes to the movement, shape and tone of the organs. When this happens it can trigger a need to return to the re-generative cycle. In other words, sometimes an hour of no-sweat Taijiquan at 8 AM can leave us wanting to get back into bed.
This can happen even if you are getting enough sleep because an individual organ may be in the process of changing. Hopefully the internal practice is improving the organ's efficiency (but as we age it is certain that eventually our organs will start to fail). When we resist the urge to sleep, we are resiting the process of re-generation.
Don't Short Change the Organs!
Whenever the seasons change or you change your internal practice, your internal organs will need a new type of sleep and rest.
If you still feel groggy after 10 hours of sleep it is because one or more of your internal organs didn't get the kind of sleep (think: shape/movement/tone) that it needed.
Note: Stillness and other "resting" practices move and shape the organs in different ways than sleep and activity. Rest, sleep, food and exercise are all indispensable