I was listening to a program from wnyc.org (NPR's New York City affiliate) called Radio Lab. The program was explaining various aspects of the purpose and mechanics of sleep. You can listen to that portion of the program here:
It reminded me of a conversation I had with Rabbi Pesach Krohn almost 10 years ago, when I once had the opportunity to spend Shabbos with him. He posed the following question (paraphrased):
Why did Hashem create us with the need for sleep? I can think of good reasons for almost every other part of human life. There are ways in which we can learn and grow spiritually from eating, marriage, breathing, talking, working, etc. But why sleep? It's simply 4-8 hours of time wasted. I understand that we cannot function without it, so in that sense it's productive. But why did Hashem create us with the need for sleep to begin with?
He ended up opining that the only answer he could come up with that was somewhat satisfying to him was that Hashem created sleep so that people would always have the ability to put a sense of finality on the Yesterday. We'd always be able to have a sense of closure and say, "What happened yesterday is over. Today I can start again. It's a new chance." If there were no sleep and one day just ran into the next, then we would never really feel that our past was behind us. Today would just be one long continuation of yesterday's failures. Sleep means a break and an opening for Teshuva.
One of the Scientists interviewed on Radio Lab explained one of the functions of sleep. The brain collects memories of virtually everything that happens during the day. Many of those memories are worth remembering and integrating and others are not. During sleep, the brain is very active, and neurologically dulls down and washes away most of the non-important or minor memories. The only memories left over are the stronger, more important ones which could not be washed away by the neurological waves that cleansed the person's memories throughout the night. This phenomenon explains why people sometimes work on mastering a skill or an information-set before going to bed, without mastering it, but upon awaking are surprised to find that they have made much progress. It is not so much that sleep helped him learn what was blocked to him before. Rather, it washed away all of the other memories that were getting in the way of the primary one.
This is a bit like what Rabbi Krohn suggested. As we go through our days, we build up not only unwanted memories of inconsequential events and information, but also thoughts, words and actions that are better left in the past. Just as sleep washes away the thoughts and memories that deserve to be left in the past on a neurological level, spiritually too sleep is needed to push the past into the past, so that we can move on with what is worth saving to a new day, unencumbered by yesterday's failings.
(Picture courtesy of azfotos.com)