The stock market is open from 9:30AM to 4:00PM EST, Monday through Friday, market holidays excepted. However, in looking at equities, one must remember, that overall it is news that moves markets, and specific news, that moves stocks. For instance, study after study has shown, that unanticipated earnings surprises whether they are positive or negative immediately impacts a particular stock price, yet most of these earning releases are provided as a press report, after hours. This, then, would imply that someone that is able to get a hold of that information and then to trade upon it, before virtually everyone else would have a material advantage over others, less informed. This, therefore, is where after hour trading or pre-market trading could potentially be an avenue to make money for an investor.
While the foregoing is definitely true, that earnings surprises or other noteworthy news such as clinical trials approved or disapproved, significant lawsuits, merger and acquisition news, and whatnot, definitely will impact equity pricing, this doesn't mean that you, as an individual investor, have the skill, discipline, and knowhow on how to profit from this. That is to say, when you are swimming with the sharks, you must determine as to whether you have true "shark-like" capabilities, or are instead, merely a pretender, and thereby thoroughly outclassed.
The value of news, and the comprehension of said news, before others are even aware of it, can be demonstrated by the Rothschild's who are said to have received news of the British victory over the French at Waterloo a day ahead of the government messengers, to which the Rothschild's rightly predicated that English bonds would become much more valuable, and subsequently made an enormous profit from it.
The fact that others can and have profited by news does not mean that you as a trader can also do so, but it does indicate that the basic premise is sound, only awaiting someone with the appropriate skill level and boldness to actuate it, as the after-hours market, is inherently and substantially far less efficient in the equitability of market pricing, as compared to normal stock market hours pricing.
In today's world, without actual access to insider information, which would probably be considered illegal, a trader cannot hope to have access to marking moving information ahead of someone else for more than a few minutes, at most. This means, for after hour traders, just being a few seconds ahead of the curve is the difference between making some money as opposed to getting played for some money, as the market is at this level, is a zero-sum game, to which, for every winner there will be a corresponding loser.
The ability to make money in after hour trading is both a skill as well as an art. It is thus, because your interpretation of noteworthy news items may be diametrically opposed to the "smart money" because they are skilled at reading in-between the lines and you are not. In short, after hour trading is not for the faint of heart, it also takes a mercenary mindset as well as a disciplined trading arm, to which most probably fall far short, while there are a few that can and will thrive.