Just before I got married I spent a Shabbos with Rabbi Paysach Krohn, who, beyond his books, speeches, and work as my son's mohel, is an amazing person even in "real life." He asked a question (to which he said he did not have a satisfying answer) that Shabbos which has stuck with me in the last fifteen years. He asked why Hashem made us with the need to sleep. He pointed out that He could have created us any way he wanted to and that he could understand good reasons for many other aspects of the human body and nature but sleep stumped him. One can point to all of the neurological benefits of sleep but all of them are built upon the fact that Hashem built us with the need for sleep. Why did He do that in the first place?
Rabbi Krohn suggested that the need for sleep could be explained by Hashem's desire that people always start each day anew and should not feel bogged down with what happened the day before. But ultimately he felt that sleep is such a waste of time and takes so many hours that this explanation by itself was unsatisfying. Why the need for 5-8+ hours every single day for the ability to refresh? Why not one hour a day or five hours once a week?
I would therefore like to share an idea of mine in response to this question:
That idea is based on the assumption that the only people bothered by how much sleep human beings need are truly great people; people who are driven to achieve as much as possible.
The need for sleep demonstrates that as great as they are, the people bothered by this question simply do not have the same priorities Hashem has. Those with this question assume that G-d, like them, wants them to achieve and do the maximum good in their lives, whether that good is in the form of any of the myriad of subcategories of Torah study, prayer/inner work, or performing acts of kindness for others. The fact that Hashem constructed human nature with this innate need to spend five hours per day "wasting time" is a message that on the deepest level, Hashem did not create a world and human beings to accomplish things in the world. He Himself, or through any of his multitude of agents, can accomplish anything He wants without us (thank you very much).
The main thing He wants from us is to want and work to serve Him. In the language of the Brisk school of Talmud study, this world is a din in the gavra, not in the cheftza; Hashem cares more about what kind of people we make ourselves than the impact we have on the world around us. As the Gemara says, "Rachmana liba bo'i," "Hashem wants the heart."
He therefore created us in such a way as to give us the message, "I want you to accomplish as much as you can in Torah and mitzvos during your time on earth, but remember that the ultimate goal is not that you accomplish things outside yourselves. Go to sleep for a few hours so that you will understand that ultimately I am responsible for the outcomes of the world; not you. I can accomplish much more than you could by other means if that were My primary goal. What I want is that you should want to serve Me, do good, and be good."
Happy to hear anyone else's thoughts on the issue. I believe that there is also a "yerida l'tzorech aliya"/"stira al m'nas livnos" aspect to it as well. I.e., even the greatest people must completely negate/destroy their current level of attainment (as opposed to merely build upon it) in order to achieve a qualitatively higher level. [Update: See more on these ideas here and here.] But I don't know that this angle adequately explains why Hashem chose to create us with the daily need for so much sleep. The same goal could ostensibly be accomplished with less sleep, less often.
Happy to hear any other input/thoughts!