What if one always finds oneself waking up spontaneously, in a moderately alert state, after about 5-6 hours' sleep and can then only achieve fitful, unrefreshi

What if one always finds oneself waking up spontaneously, in a moderately alert state, after about 5-6 hours' sleep and can then only achieve fitful, unrefreshing sleep for the rest of the night? In my youth, in the absence of an alarm, I would consistently get 8-9 hours of deep, unbroken sleep, but since my early 20s, I've found it impossible to get more than about six hours in one block. If I do succeed in returning to sleep after a premature awakening, I'll wake up again no more than 30 minutes later from what feels like a very non-restorative light sleep. Schedule permitting, what seems to work best for me is a biphasic routine where the bulk of my sleep, typically about 5-6 hours, is had at night together with a short siesta in the afternoon, usually after lunch.