Taken from its page on Wikipedia --
Non-24-hour sleep-wake syndrome is a rare and chronic circadian rhythm sleep disorder, classified within Chapter VI, Diseases of the Nervous System, in the ICD-10. It can be defined as "a chronic steady pattern comprising one- to two-hour daily delays in sleep onset and wake times in an individual living in society." The pattern of delay persists literally "around the clock," typically taking a few weeks to complete one cycle. It affects less than 0.05% of the population (fewer than 1 in 2000 people), which means that it is classified as a "rare disease." ...
I has a name for it! Now, if only I can find a way to fix it.
For further amusement, I find this other "harmless" disorder that travels with the above. Which I think I also have (in the mildest form -- occasional bursts of VERY loud ringing) --
Exploding head syndrome is a condition that causes the sufferer to occasionally experience a tremendously loud noise as originating from within his or her own head, usually described as the sound of an explosion, roar, waves crashing against rocks, loud voices, or a ringing noise.
I've not noticed the symptom while sleeping ... but it could explain some of my sudden-waking experiences of the past. I've *definitely* noticed it while awake.