Flushing brain of waste material may be one reason we sleep

DEAR DOCTOR K: A friend told me that the reason we sleep is to "flush out" our brain. What is this all about? Is this the reason we sleep?

DEAR READER: Many readers have written asking why we sleep, and we discussed it in yesterday's column. Today we will talk about the recent study that you are asking about, which suggests that one reason we sleep may be to flush out the brain.

For those who didn't read yesterday's column, a quick summary. There is evidence that during sleep, our mind and body benefits in several ways. Perhaps most obvious, our muscles get a rest. The fortunate exception is the special muscle that is our heart. We don't want it to quit pumping -- ever!

Do our other organs need a rest? Most of them keep working while we sleep. But they all sustain some wear and tear from their work, during both the day and night. Therefore, they may well need to repair certain injuries that have occurred during the day. During deepest sleep, hormones are released that stimulate tissue growth and muscle repair. And your immune system becomes primed to defend itself against infection the next day.

Does our brain need the rest? Brain waves are very active during sleep, but our brain is doing different things during sleep than when awake. For example, it dreams. Which raises the question of why we dream. Sigmund Freud thought dreaming was our way of dealing with hidden conflicts, desires and fears. He thought we needed to sleep because we needed to dream.

In the past 10 years, scientists here at Harvard Medical School and elsewhere have published a great deal of evidence that dreaming (and, hence, sleeping) is important in memory and learning.

Now to your question. A truly remarkable study was published two years ago. To explain what was found, I first need to explain some concepts. Our brains are filled with billions of different kinds of cells. Between those cells is space through which fluid passes. It's as if there are little rivers running through the brain.

The study was of mice, whose brains are not terribly different from ours in many respects. (Granted, their brains couldn't come up with the theory of general relativity. But their brains are much better than ours at smelling cheese.) The study found that during sleep, the little rivers that run between brain cells increase greatly in size. As a result, many substances made by brain cells (including cell waste material) get flushed out of the brain much more efficiently.

For example, two molecules called beta-amyloid and tau are flushed out of the mouse brain more efficiently during sleep. These two molecules build up in the brains of people with Alzheimer's disease.

So maybe your friend is right. One important purpose of sleep may be to clear potentially toxic material out of the brain. And perhaps we feel restored in the morning for that reason. It is an arresting, and plausible, possibility.