Does anyone else get this? They find themselves in a habit, where for a few days running, they wake up at roughly the same time each night and need to go to the toilet?

Maybe it's not even needing to go to the toilet, but simply looking at the time and realizing that you woke up at this time last night, and the night before, and the night before again. Then, having noticed this, it's quite difficult to get back to sleep...

Sure, it may be partly confirmation bias where we only happen to notice and remember the act when there's a perceived pattern. But my question, and my story, go a touch further...

For three nights in a row I got up thinking it must be quite close to morning (around 4am-ish...) only to find it's actually ~1:30am. That is, I woke up thinking I'd been asleep for 5 or 6 hours, when I'd only actually been asleep for 90 minutes. On the fourth night - having noticed this pattern - I got up expecting this strange sensation, only to be completely baffled by the concept of time. It's kind of hard to explain, but I could equally believe that it was some time in the mid-afternoon, as I could that it was 2am. I was genuinely confused. Naturally, I just went back to sleep.

Problem. Solved.

This got me thinking. How do we measure time when we're asleep? My system had clearly gone (temporarily) haywire. Without a clock I would have been completely wrong regarding time. Under different circumstances I may not even have bothered going back to bed. However, If I'm really trying to understand the phenomenon, then the first logical question is 'how do we measure time when we're awake?' and then 'does it differ from when we're asleep?'. This, however, was not the way I tried to research the problem. I wanted a quick meta-review that told me 'time is understood by x process when asleep'. Well, no such thing exists, apparently. Not that I could find, anyway.

So before asking the logical question 'How do we understand time?' I want to throw it open. How do you think we process time when we're asleep?

I asked my girlfriend and her reasoning happened along these lines (I'm paraphrasing...):

We determine time elapsed only when we wake. We do this by implicitly assessing features relevant to tired/wakefulness, e.g. how 'rested' we feel, how comfortable we feel, etc. We probably wake up frequently during every night's sleep, even though we may not remember it, and calibrate our judgements regarding sleep-time during these periods. 

As I said, I couldn't find any research on the cognitive / physiological processes that govern our time perception during sleep. All the literature relates to insomniacs and methods for dealing with that particularly problem.

So what are your thoughts? How do you think we figure it out, and what personal experiences lead you to think this way (if any)?