Much slumber.
Why do prophets and apostles consistently counsel us to get "enough sleep"? Well, here are a few "physical" and "spiritual" reasons.
The Physical Reasons
Though scientists are not exactly certain why we need sleep, they do have several theories.
http://www.bbc.com/future/story/20120228-why-do-we-need-to-sleep says:
One theory that has emerged in recent years is that sleep helps us to process and consolidate new memories. Our memory system is a psychological wonder, and several studies have suggested that sleep provides some behind-the-scenes maintenance.
One school of thought is that sleep aids our memories by refreshing and reorganising them without interfering with our waking thoughts. Evidence comes from several studies using methods that record the brain directly. For instance, when rats were trained to find their way around a maze, their brains produced the same activity patterns during sleep as when they had carried out the task – suggesting that the brain was reconstructing the experience.
From this we also gain an important insight into the fascinating phenomenon of dreams. These crazy adventures our minds have while we are sleeping may be a product of our memories randomly activating so as to keep them fresh, and of the mind seeking connections between all the things we have recently experienced. This could also explain why hallucinations accompany sleep deprivation. Without the opportunity to reorganise our memories during sleep, dreams intrude into our waking lives, causing difficulty in distinguishing our inner lives from reality.
So, not only does sleep help give our body a chance to have a break, repair, and rest...it also can help with storing memories and sorting through them. Sleep is a vital part of our lives, just like eating, drinking, and breathing. Our body cannot fully function, nor our brain properly think or command, without a little bit of nighty-night.
The Spiritual Reasons
Elder Richard G Scott, in this previous April 2012 conference, said, "Revelation can also be given in a dream when there is an almost imperceptible transition from sleep to wakefulness... Exercise, reasonable amounts of sleep, and good eating habits increase our capacity to receive and understand revelation. We will live for our appointed life span."
How many times in the scriptures have we heard about dreams, visions, and so forth. For example, in the book of Genesis, we learn of Joseph and the Pharaoh. Pharaoh dreamed about the cattle and the corn. Later on Joseph was able to decipher the dream and help save Egypt during famine. (see Genesis 41.)
Obviously, sleep is important. I think sleep can be a great tool for God to use in order to give us special warnings, visions, and promptings.
President Monson has said:
The Apostle Paul declared, “Know ye not that ye are the temple of God, and that the Spirit of God dwelleth in you? … The temple of God is holy, which temple ye are” (1 Corinthians 3:16–17). Nutritious meals, regular exercise, and appropriate sleep are necessary for a strong body, just as consistent scripture study and prayer strengthen the mind and spirit.
Modern-day LDS temples even take "naps." They are closed for cleaning, renovation, and so forth. Maybe sleep is like that for the body. We are better fortified when we wake up, we are fresh and ready for a new day, and we are preparing ourselves to receive revelation.
