Stages Three and Four of Sleep

We hope you found last month’s look into stages one and two of sleep to be insightful and enjoyable. If so, then we hope you’ll join us as we look at the deep sleep stages.

Unlike the first two stages that are for shifting through the day’s events (often in the form of dreaming), the coma-like sleep that is experienced in stages three and four are there to provide your mind with some much needed “physiological housekeeping” (Finkel, 56).

The behavior of sleep dates back to single-celled organisms, which can be studied in today’s one-celled organisms such as plankton and yeast. Even they display active periods followed by periods of rest. “‘Being awake is demanding,’ says Thomas Scammell, a neurology professor at Harvard Medical School. ‘You’ve got to go out there and outcompete every other organism to survive, and the consequences are that you need a period of rest to help cells recuperate’” (Finkel, 66).

For humans that period of rest and recuperation happens primarily during the deep sleep that occurs in stages three and four. The difference between the two stages can be seen in the study of delta waves measured on an EEG. Some scientists find the two stages so similar that they consider them one stage. This is because stage three has delta waves present less than half the time while stage four has them over half.

It is during deep sleep that cells begin to produce more growth hormone, which is needed for repairing and strengthening bones and muscles. Because of this period of recovery, which extends to the maintenance of a healthy immune system to maintaining a proper body temperature and blood pressure, our bodies are able to recover from injuries. It also aids in regulating our moods as well as reducing the risk for dementia.

This period of deep sleep is so essential, that animals have been observed to die from sleep deprivation before starvation. Still, scientists warn that too much sleep can be dangerous. In stage four, “‘You’re talking about a level of brain deactivation that is really rather intense…Stage 4 sleep is not far removed from coma or brain death. While recuperative and restorative, it’s not something you’d want to overdose on,’” says Michael Perlis, the director of the Behavioral Sleep Medicine program at the University of Pennsylvania (Finkel 66). At most, our brain only lets us stay in stage four for about thirty minutes at a time.

Join us next month as we delve into the workings of REM sleep.


Finkel, Michael. “Want to Fall Asleep? Read This Story.” National Geographic, Aug. 2018, pp.40-77.

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