When you buy a stock, you should have a good reason for doing so and an expectation of what the price will do if the reason is valid. At the same time, you should establish the point at which you will liquidate your holdings, especially if your reason is proven invalid or if the stock doesn’t react as expected when your expectation has been met. In other words, have an exit strategy before you buy the security and execute that strategy unemotionally.
By understanding your risk tolerance, you can avoid those investments which are likely to make you anxious. Generally speaking, you should never own an asset which keeps you from sleeping in the night. Anxiety stimulates fear which triggers emotional responses (rather than logical responses) to the stressor. During periods of financial uncertainty, the investor who can retain a cool head and follows an analytical decision process invariably comes out ahead.