Like you I am a lifetime short sleeper. I also tried melatonin and sustained release melatonin at bedtime and it made no difference; I had abrupt cortical awakenings in the middle of the night and unable to get back to sleep to sleep the prescribed 7 to 8 hours even with CPAP and normal weight and lab values and all recommended sleep hygiene.
I am not sure that the short sleeping has actually had any adverse health effects, but it is socially unacceptable since I also generally fall asleep too early (extreme "lark" when others are "owls"). What has helped is realizing this syndrome is not my fault but a probable undiagnosed advance phase circadian sleep disorder (phase shorter than solar cycle, advancing each day). I can try to delay my circadian rhythm including sleep cycle by a combination of interventions. I now take the melatonin if I awaken early, even if I can't get back to sleep. Sleep doctors don't agree on this, as they think it might be dangerous driving and promote narcolepsy, but it is worth a try if only based on my anecdote. So instead of no blue or cyan light after sunset and melatonin at bedtime, use melatonin in the middle of the night and no blue light until sunrise. Also to delay the sleep cycle, use caffeine later. Caffeine and melatonin might protect against Alzheimer's, but I haven't seen a human study. https://www.ncbi.nlm.nih.gov/pubmed/22959965
Circadian sleep disorders might be associated with variants in the PER clock gene. I have a rare variant in the PER3 gene that has been studied, aiding my amateur self-diagnosis. http://www.neuron.illinois.edu/files/U3_L4_CheckpointB_FamilialAdvancedSleepPhaseSyndrome.pdf
Of course, phase of circadian rhythm (less than or more than the solar cycle) involves more physiology than just sleep, but is related. (Seasonal Affective Disorder and extreme jet lag are common if the cycle is not 12/12hrs). It is possible of course that your sleep disorder has nothing to do with circadian rhythm and if you are concerned about your sleep you should see a sleep physician.
Other circadian sleep disorders include delayed phase, and free-running cycle common to blind people, and there is much information online. Formal diagnosis is a bit difficult since the usual way to assess circadian rhythm is core temperature requiring a rectal probe, along with EEG polysomnography, and I haven't been able to duplicate that at home. Help!