What you are referring to is intra-day noise. This effects short term or intra-day traders. Mid to long term traders generally avoid this intra-day noise to ride the trends.
When you look at the performance of the stock market at the end of a trading day it can be hard to work out why shares have either risen or fallen in value.
Broadly speaking, share prices are influenced by news or information: new data on employment, manufacturing, directors dealings, political events or even the weather, all kinds of news can influence the way shares move.
The health of the economy has a fundamental influence on share prices because it is ultimately responsible for driving company profits. Broadly speaking, if the economy is growing, company profits improve and shares will become more highly valued. If the economy is weakening, company profits will fall and share prices will go down.
Investors look at a vast amount of data to try and work out what is going to happen to the economy and shift their portfolios before the events occur. This is why you will often see markets move well ahead of an actual event occurring. You may, for example, get little reaction from the stock market when interest rates rise. This is because investors have already anticipated the shift months in advance and adjusted their portfolios beforehand.
You can usually assume that the stock market will anticipate moves in the economy by around six to nine months. So if you want to stay ahead of the game you will need to follow price data as closely as the professionals.
Investor sentiment can lead to irrational buying or selling of shares and result in bull and bear markets. A bull market is when share prices rise while a bear market is when they fall. In the technology boom of the late 1990s, for example, investors paid extremely high prices for shares and ignored traditional valuation measures, such as P/E ratios. This carried on until 2000 when investors belatedly realised these shares has risen too far and resulted in a three year bear market in shares.