Students and exhausted workers have been abusing stimulants for decades. But these days, if you're keen to feel sharper, and work for longer, the drugs available are more effective than they once were - and less likely to make you feel awful afterwards. No wonder then that the abuse of pills such as Ritalin and modafinil is becoming a major problem. Katharine Hibbert reports on the rise of cognitive-enhancing drugs - and doctors' fears over their long-term safety.