Question: I have been told that if a person sleeps too much at one time, say for 12 hours, that they begin to dream too much, and then start burning the energy that they would need the next day. Does this make any sense, and is it true?
Thank you.Kevin’s ThoughtsI’m glad you’ve asked about this, because it is definitely a commonly held misconception that people can sleep too much, in the basic sense of the phrase.In the vast majority of occasions people sleep an unusually large amount of time (like the 12 hours you mention), they do so because their body calls for them to in order to make up a sleep debt that has been previously accumulated (See this page for a refresher on the concept of sleep debt).

A simple illustration of what I mean: Say you have a daily sleep requirement of 8 hours. You sleep only 4 hours on Tuesday night, thus accumulating 4 hours of sleep debt. On Wednesday night you go to sleep at 9 PM and sleep straight through to 9 AM. By obtaining 12 hours of sleep you have eliminated your 4 hours of sleep debt from the previous night and your body is more or less back to the level of alertness and functioning capacity it was at before you lost sleep on Tuesday night. The extra dreaming and REM sleep cycles has, to the best of my knowledge, not affected this.A more detailed explanation of why this is the case and why it is difficult to sleep too much, with a few more terms thrown in, follows below:Our bodies’ desire to sleep is monitored by a homeostatic process (known as the opponent process model, where sleep debt and clock-dependent alerting factors determine whether or not we are able to sleep, or whether or not sleep is helpful). As such, the ability to sleep 12 hours at one time more or less signifies that the body can use at least those 12 hours to restore peak alertness and function, as demonstrated in the example above.Because of this homeostatic process, biologically speaking it is impossible for most of us to sleep too much. Of course, complications with this can arise when other factors thwart the homeostatic process, such as depression or the extremely rare Kleine-Levin syndrome (also known as Sleeping Beauty syndrome).But the takeaway point remains that in an otherwise healthy person, it is virtually impossible to sleep too much, from a biological standpoint.