Why Do We Sleep?

Why Do We… is a biweekly column written by Nicholas G Norwitz ’18.

“Seven billion afflicted by loss off consciousness syndrome – no human is spared.”

Perhaps this isn’t a breaking news headline. You likely know that we (should) spend about eight hours of every day sleeping. But for a behavior that consumes a full third of all our lives, sleeping is a rather poorly understood behavior.

I’m sure you’ve heard a lot of theories about why we sleep, such as energy conservation, tissue restoration, or memory consolidation. While sleeping does serve each of these purposes, many scientists believe that these are secondary benefits of sleep. What is the primary reason we sleep? Why did sleep evolve? To answer this question, we must first ask, “what are the two major problems that any metabolically active organ must solve?”

The answer is to acquire energy and to dispose of waste.  Since the brain consumes a quarter of the body’s fuel and produces a proportional amount of waste, it must have robust energy supply and waste removal systems. In most of the body, energy is supplied by the blood vessel system and waste is removed by the lymphatic system. The brain has an extensive blood vessel system. However, the brain is so jam packed with its 100 billion neurons and blood vessels that there is no room left for a lymphatic system!

So how does the brain get rid of the immense amount of waste it produces?  Enter sleep! When we sleep the brain demands less energy. Therefore, the brain’s blood vessels can constrict, creating channels that perfuse the brain. The brain then pumps cerebral spinal fluid through these channels, flushing out the day’s metabolic waste.  We all know that a good night’s sleep clears the mind. Now you know it does so literally as well.