I've read about anecdotal evidence that human brain has a fairly good internal clock that can be used to judge the amount of time that has passed. I'm wandering if there are any mechanisms in the human brain by which the brain can make adjustments to biological processes, based on the expectation of getting so many hours of sleep in the upcoming night. In this case, an alarm clock is not used.
For example, a person typically sleeps for 8 hours a night. On one day, that person realizes that he/she will sleep for only 5 hours that night, has there been any studies that demonstrate that the person's sleep would be "different" from the regular sleep? Will the person's sleep on that night be just a "cut short" version of regular sleep, or will the sleep be restructured to accomodate sleep time restriction?
For example a person's normal sleep cycle is 90 minutes long, so in 8 hours there would be about 5 complete cycles. Would there be any observable adjustments to the sleep cycle length or the amount of deep/REM sleep within the cycle, if a person expects to sleep only 5-6 hours or 10+ hours?
Thank you for your input!