Science in the Torah

The Ideal Amount of Sleep - Rabbi Zamir Cohen

What goes on when we sleep? Is it just a matter of letting our body rest? Prof. Robert Stickgold: "During an eight hour night, there are two main critical stages during which the brain undergoes both physical and chemical changes". What do our Sages say?

The Ideal Amount of Sleep - Rabbi Zamir Cohen

The number of hours people need to sleep obviously varies from one individual to the next. However, is there a recommended amount of sleep that could engender tranquility, clarity of mind and the easy absorption, comprehension and recall of material that has been studied?

A recent study, undertaken by a research team under the direction of Robert Stickgold – a professor of psychiatry at Harvard University Medical School and one of the most respected experts on sleep in the world – suggests that eight of hours of sleep is the ideal amount required to achieve the goals listed above. In no case should it be less than six hours, whereas more than eights hours of sleep a night is a waste of time!

Prof. Stickgold explains that during an eight hour night, there are two main critical stages during which the brain undergoes both physical and chemical changes:

  • The first part of the night.
  • The early morning, during the last two hours of sleep.


The interaction between these two stages strengthens and establishes the memory. In the first stage of sleep, information flows from the memory center of the brain, the hippocampus, to the cerebral cortex. Afterwards, over a period of four hours, the brain slowly distributes the new information that was studied and acquired during the day to the appropriate locations amidst the neurons of the central nervous system. “The process is similar to the period required for dough to rise,” says Dr. Stickgold.

In the second stage – known as the active dream state and the period in which sleep ends – rapid chemical processes also take place. The hippocampus disconnects from the cerebral cortex and the brain once again processes the information that had undergone previous refinement, strengthening the connection between this new material and the nerve cells at all levels of the memory system.

Prof. Stickgold surmises that his study has important repercussions for students and others seeking to acquire new information or skills in various areas of endeavor. Namely, that the acquisition and application of new knowledge depends much more on the right amount of quality sleep than on study techniques or time invested by the student – and it is even more important than his or her level of intelligence.

It is worth noting that a recent publication of this study merited headlines in the press around the world, particularly in scientific journals.

We now turn to the Mishneh Torah, the classic text of Maimonides, which delineates all of the laws of the Torah and their relevant customs. Here Maimonides states simply – amidst other remarkable pieces of advice concerning physical and spiritual health – that eight hours of sleep is sufficient for people, while more than that provides no benefit to a person’s health:

“The night and the day together equal twenty-four hours. It is enough for a person to sleep for a third of them, which is eight hours.”

It is interesting to note that certain kabbalistic masters found an allusion to this principle in a biblical verse: “For now I have lain down and should be quiet, I slept and then should be rested” (Job 3:13).

The numerical value of the letters of the Hebrew word “then” – az – is eight: aleph(1) + zayin(7) = az(8).

Hence the verse can be paraphrased: “I slept ‘eight’ (az) and then should be rested!”

According to Jewish mystical teachings, all chemical and physical phenomena that occur in the human body – including processes that take place during sleep – are rooted in the spiritual realm. Hunger, for instance, which appears to be a completely physical phenomenon, actually derives from the nefesh (the lowest of five levels of the soul: nefesh, ruach, neshamah, chayah, and yechidah), which oversees the needs of the body. Mere flesh alone, without nefesh, does not have feelings or desires. Likewise, physical nourishment occurs by virtue of the spiritual spark hidden within the food, which provides life and sustenance to those who consume it. 

Adapted from The Coming Revolution by Rabbi Zamir Cohen
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