Most people have a Circadium Rhythm which is optimal for their body. This involves a lot of different processes like hormone release, core body temperature levels, response to light levels and a whole bunch of other stuff best explained by someone better versed in medicine than I.
The majority of the population (around 70%) fall into the typical 11–12pm sleep and 8 am waking cycle which you described in your first example.
When that is disrupted for any reason, your body gets thrown out of rhythm and is not operating at its normal peak efficiency. The amount of sleep is not the issue. It’s WHEN that sleep occurs in relation to your body and mind’s rhythm relative to other body functions.
I happen to be in that other unlucky 30% and don’t begin to approach optimum functioning until around noonish or so. It’s not that I can’t function in early am hours. It’s just that I don’t function as well. Grogginess, crabbiness, general unease etc.
It seems that your bod’s natural rhythm is in the first pattern you described. When you force it out of that pattern, you don’t feel right.
It wasn’t until I read a thorough explanation of the importance of Circadian rhythms that I finally understood why I thought I was an insomniac for so long. Once I stuck to my particular body’s preferred natural rhythm, (instead of society’s dictates) everything improved greatly.
Do a little reading up on the subject and try to follow your particular body rhythms and I think you’ll feel better for it in multiple ways.