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!

  • $\begingroup$ From my personal experience, I can tell you one thing for sure: If, before going to sleep, I know that I have to wake up early or sleep less, and I think about this positively and say something like "ok, buddy, this time you've got just 4 hours to sleep. we need to get as much rest as possible. prioritize sleep over all other for these 4 hours. and don't get surprised being awoken after 4 hours of sleep. so make your cycles well fit this period to get as much rest as possible." - and play with these thoughts and anticipations, you know, it works. Maybe its more about auto-training, but still.. $\endgroup$
    – noncom
    Commented Mar 26, 2015 at 17:45

1 Answer 1


I haven't heard of studies like the one you're proposing, but you can test this hypothesis on yourself with this app.

Knowing you need to sleep less than your usual amount will probably affect the quantity and quality of your sleep, but not because of an "internal clock". Most likely, whatever you need to wake up early for will cause you some anxiety, which will affect both sleep duration and quality.

  • 1
    $\begingroup$ you are suggesting my own app... :) $\endgroup$
    – Alex Stone
    Commented Mar 20, 2013 at 13:44
  • $\begingroup$ :D It's a nice one. $\endgroup$
    – Drosophila
    Commented Mar 20, 2013 at 16:58

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