I've experimented with all the commonly suggested stuff: blue-light blocking, sublingual melatonin (dosed appropriately and timed according to the phase response curve), exercise, meditation, etc., simultaneously and in isolation, and nothing extended my sleep significantly.
I suspect if I went through a grueling few weeks of sleep restriction, I would eventually see an increase in the duration of my sleep, but I'm content for now to stick with my biphasic protocol.
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.
I suspect if I went through a grueling few weeks of sleep restriction, I would eventually see an increase in the duration of my sleep, but I'm content for now to stick with my biphasic protocol.