Neglecting good sleep as a way of doing mathematics is not a good idea. To do mathematics, you need to get enough rest. As to the number of hours per day, it is impossible to count this. When you think about a mathematical problem, you think about it all the time, including when you are asleep. It is true that mathematicians who work more do tend to achieve more, and those with the very top achievements do tend to work really a lot. But no amount of hours spent per day sitting at the table guarantees anyone anything in mathematics. The key word is not "long hours", but "dedication".