What is the physiological function of sleep and is there such thing as “good sleep” and “bad sleep×´?
The Gemara on amud beis relates to is an incident when Rabbah was dozing off and his student Abaye attempted to get his attention. The Gemara notes that dozing is distinct from sleeping. When dozing, one can answer when called, be reminded and confirm certain details but not engage in thoughtful discussion. When asleep, a person is not even able to respond in any logical manner.
Sleep researchers also have a taxonomy for sleeping and divide sleep into four stages, which cycle through anywhere from 4-6 times a night, each cycle lasting 90 minutes. That is, in a 90 min cycle, a person goes through all four stages, and this is repeated 4-6 times. This is why in most adults, 6-8 hours of total sleep is the optimal to allow for four cycles of four stages.
Stage one is the lightest stage and represents the beginning of sleep, which usually lasts 1-5 minutes and breathing and muscle tone are near normal with somewhat slowed brain activity. This is probably the stage of “dozing” that the Gemara was referring to.
Stage two is the beginning of a deeper sleep, with markedly slower breathing, heart rate and brain activity. This lasts for approximately 25 minutes.
Stage three is the deepest sleep in the sense of the slowest breathing and slowest brain activity. It is most difficult to wake people from this stage and even when they wake up they tend to be groggy and foggy for up to 30 minutes. This stage lasts from 30-60 minutes. The physiological purpose of this stage seems to be to allow the body’s muscle joints and tissue to rest, regenerate and heal, and to strengthen the immune system. This is why sleep is good when you’re not feeling well. Think of it as kind of a miniature, medically-induced coma.
The fourth stage is known as REM sleep. It is named as such because it is noted that despite the rest of the body being in a low activity state, the eyes behind the eyelids move rapidly, and there also is almost as much brain activity as during wake time. This is typically where dreams occur, and the stage lasts for about 10 Minutes but toward the end of the night in the final cycle can even be an hour. The purpose of this stage of sleep seems to be where the brain recharges allowing for mood stabilization, creativity and memory consolidation.
This is why physical conditions such as sleep apnea can cause harm, even if technically, the person is sleeping for 8 hours. In sleep Apnea as the person enters the deeper stages of sleep (3 & 4), the throat muscles collapse and obstruct the airflow, causing the person to wake into a lighter sleep. This prevents a person from entering the deepest levels of sleep and they lose out on the neurological and physiological restorative functions of sleep. In addition, if a person wants to take a “nap” to make up for lost sleep, a short nap may work in terms of satisfying the biological urge and pressure to sleep much as a candy bar can stave off hunger. So you may not feel the need to sleep as much, just like you won’t feel so hungry after eating junk food. However, in both cases, real nutrition is not achieved. To take a nap that actually restores lost sleep one would need to go through the four stages, which typically takes 90 minutes.
Source: https://www.ncbi.nlm.nih.gov/books/NBK526132/
Translations Courtesy of Sefaria, except when, sometimes, I disagree with the translation 
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