MadSci Network: General Biology |
Marvin,
Good question! In short, what we think is happening is that various energy byproducts are building up in our brains over the day, and they get cleared over the night. It's those byproducts that make us sleepy, so when they are cleared out, we feel refreshed in the morning. I'll elaborate.
First, I'd like to redefine some terms: energy versus alertness. I think what you're describing would more accurately be described as alertness. When I think energy, I think of fuel (like gasoline). Food provides the fuel that gives us energy to move our muscles and make our body systems function. The source of energy in our cells is called Adenosine triphosphate, or ATP. But I'll get to more about ATP later.
The reason that I make the distinction is that our bodies are running low on fuel in the morning, as we haven't eaten since the night before! Therefore, alertness doesn't depend on the materials for energy in our body, but on something else.
The question is: what is this alertness? The website http://www.alertnessmatters.com/ defines it as being: vigilantly attentive; or mentally responsive or perceptive. I think these are pretty good terms, and they describe how I feel after a good night's rest. But what does an alert brain look like? And why does it become less alert throughout the day and become refreshed by sleep?
I will confess that we only have partial answers. We really don't know why we need to sleep to restore alertness; we only know that we have to. But we have some ideas, and we do know there are ways that we can modify a person's alertness, at least temporarily.
You may be very familiar with one way to increase your alertness: The caffeine in your coffee/tea/soda. Caffeine acts by blocking adenosine receptors in the brain (the same adenosine as mentioned above in ATP). People have discovered that the amount of adenosine in the brain increases as the day goes on, and that this adenosine makes you sleepy and less alert. Caffeine makes you alert by blocking the action of that adenosine.
But what is adenosine doing in the first place? After ATP is used to make
energy, adenosine is left over as
a byproduct. And it turns out that adenosine acts to quiet down neurons. If
you add adenosine directly to an
animal's brain, it gets sleepy.
So the logic goes like this: after a brain has been super active all day,
using lots of energy, all this
adenosine builds up and ends up turning the brain down a bit; we become
sleepy. At night, our brains don't
shut off, but the pattern of activity is drastically different, and it is
thought that a sleeping brain uses
less energy than during the day. Since we are using less energy, the
adenosine doesn't build up, it can be
cleared, and we can wake up with a low adenosine level, refreshed.
The picture I paint is a little too simplistic - there's a lot of devilish details that tell us that adenosine isn't the only thing responsible. There are a lot of pieces to the puzzle. But it generally makes sense - adenosine is left over after we use our energy stores, and adenosine acts to shut our neurons down and makes us groggy. At night, we don't use as much energy, the adenosine is cleared away, and we awake refreshed and alert!
I hope that makes sense. I got most of my info from two science papers:
Benington & Heller, Progress in Neurobiology, Vol 45, pp 347, 1995 - Restoration of brain energy metabolism as the function of sleepSearching for adenosine and sleep pulls up some interesting stuff. Google holds even more!
Saper, Scammell, & Lu, Nature, Vol 437, pp 1257, 2005 - Hypothalamic regulation of sleep and circadian rhythm.
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