Sleeping while on duty is a misconduct. You can take disciplinary action against the employee. For that what is the need of a Standing Order. It is true, if you have certified Standing Orders, the same would help you to decide the punishment but absence of standing order does not put you in trouble when you want to initiate an action against an employee. Only thing that you should follow is that before awarding a punishment the accused employee should be given an opportunity to be heard. In the case of sleeping while on duty, the same should be followed. He might have had a sleepless night preceding to the working hours due to some genuine reason, say, his daughter was hospitalised and he had to spent the night at hospital to take care of her without sleeping. Hence ask him what is the reason and then take a call. Normally, for first time offence only warning will be awarded and no punitive punishment shall be given.