I know what you're thinking. As a value-seeking investor and longtime adherent of the Buffett and Graham school of investing, I must be using this article as another opportunity to say that Berkshire Hathaway (NYSE: BRK-A) (NYSE: BRK-B) is a must-have investment.

But I won't -- not today anyway. Sure, an investment in Warren Buffett's lifelong masterpiece offers investors exposure to a wide catalog of businesses ranging from insurance to underwear to candy. And, at least for the near term, you've got the finest investing craftsman running the show.

But Berkshire is not the one. Nor is it the mighty Coca-Cola (NYSE: KO), or some emerging-market gem like India's Tata Motors (NYSE: TTM), which is aiming to revolutionize the automobile market with the $2,500 car. The stock you need to own is much more valuable than these fine companies. And you don't need to be a gifted investor with a mind like Buffett to realize its value.

In fact, the stock I am referring to will put you to sleep.

Sweet dreams
Investing in the stock market is guaranteed to take you on an emotional ride. Before an IQ of 130 or an MBA, any successful investor must have a strong temperament. A high IQ doesn't help you when your investment is down 50%, but a strong temperament gives you the ability to sit still -- or even to back up the truck. An Ivy League MBA doesn't help you when the "smart money" on Wall Street is betting against you, but a strong temperament helps you ignore the noise.

If your investments keep you guessing all night long, you really don't understand the investment, in which case you need to cut your losses and move on. At some point, every business will have its upswings and downswings. How you behave during the tougher times is what counts. For some companies, the downswing is a natural part of the business cycle.

For example, consider refiner Frontier Oil (NYSE: FTO). Like all refiners, Frontier's getting hit by rising oil prices. If the price of a refiner's input -- oil -- rises without a corresponding rise in the price of its output -- fuel -- then the refiner's profits decline. No matter what they do, quality refiners like Frontier will suffer from a widening of the oil-fuel price gap. And if oil prices continue climbing, most refiners will find it difficult to increase earnings.

Yet Frontier stands out from its competitors in its ability to refine heavier sour oils, which sell for much less than the sweet stuff that we hear about every day. And over the long term, the need for fuel isn't going away. So with lower input costs -- and a solid balance sheet with no net debt -- Frontier looks better suited than its competitors to weather the storm.

Know your investment
If you're trying to jump over 7-foot hurdles and invest in a subprime lender -- or any of the other financial companies with balance sheets harder to decipher than a chemistry experiment -- you'd better really understand it. If you don't, I guarantee you the volatile stock price will keep you second-guessing yourself way past midnight.

So to keep your emotions in check, the only investment you should make is the one you can understand -- because that allows you to slumber peacefully. Don't just take it from me. None other than value investor extraordinaire Seth Klarman of the Baupost Group touts the benefits of investing so you can sleep well.

Emotions are hazardous to your investing health
Investing should improve your life over the long term, not wreck it in the short term. Avoid investing in companies that you can't understand, as the slightest hint of trouble will likely keep you guessing around the clock. Having your emotions dictate your investing decisions is a recipe for disastrous returns. Any investment that promotes emotional thinking over rational thinking should be ignored or sold.

I doubt Bill Miller is losing sleep over Countrywide (NYSE: CFC) or Bear Stearns (NYSE: BSC), but very few investors have the mind of Miller. Yet I'd bet that even Miller experienced some agitating thoughts. And if it can happen to him, it can definitely happen to you.

It's far better for you to take your losses, sleep well at night, and start over with those businesses that let you keep your emotions in check -- those you understand. Your profits and dreams will be a lot sweeter.

Further Foolishness:

Coca-Cola and Berkshire Hathaway are Inside Value recommendations. Berkshire Hathaway is also a Stock Advisor pick. See how we forget our emotions and stick to our knitting by taking a 30-day free trial today.

Fool contributor Sham Gad runs Gad Partners Funds, a concentrated, value-centric investment partnership modeled after the 1950s Buffett Partnerships. He has a stake in Berkshire Hathaway. He has no emotions toward stocks -- only at the golf course. The Motley Fool owns shares of Berkshire Hathaway. Tata Motors is a Global Gains recommendation. The Fool's disclosure policy won't make you cry.