How much sleep should we get and why aren't we getting it?

Some ABC's and Z's in just a few minutes.

First up today, Wall Street.

Yesterday, the Dow Jones Industrial Average closed the day above 15,000.

That's significant, but why?

But for starters, it's never happened before.

And that number, 15,000 comes from a complicated formula based on 30 stocks that are part of the Dow Jones Average.

Higher number equals higher value for those stocks.

Those companies represent a variety of industries.

The Dow gives an idea of how the stock market is doing and the stock market is one indicator how the U.S. economy is doing.

So, when the Dow reaches a new record,

it means investors are feeling more positive,

experts seeing this as a sign that the U.S. economy is improving.

Next, President Obama welcomes South Korean president Park Geun-Hye to the White House yesterday.

This is her first visit to the United States as South Korea's leader.

The countries have been allies for decades,

and as part of that relationship, the two countries run joined military exercises every year.

A major part of this year's exercises wrapped up last week.

Something else the U.S. and South Korean have in common.

both countries have a tense relationship with North Korea.

That country recently made threats against South Korea and the U.S.

The tension has calmed down a little bit,

but North Korean gave a new warning this week about military drills happening off the West Coast of the Korean Peninsula.

See, if you can I.D me:

I'm a country that has one of the world's only communist governments.

I'm the fourth largest nation in size,

but I have the world's second largest economy.

I'm first when it comes to population.

I'm China, and I became the world's second largest economy in 2010.

China took over that number two economic ranking because its economy grew.

It's something you hear U.S. politicians talk a lot about economic growth.

Usually, it's considered a good thing,

when a country's economy grows too much too quickly,

there's a chance it can start supplying things that aren't in demand.

Ivan Watson gives us one big example。