Dear Ramesh,
Notwithstanding the completion of the targets, theoretically speaking, no worker can sleep while on night duty. If he has completed work assigned to him then he should assist someone else or he should be given another task. By the way, what the supervisor or the production manager was doing when the worker slept? Why some alternate work was not assigned to him?
Anyway, this is the internal matter of your company. We the members of this forum are outsiders and cannot provide the solution. This is because we do not know how far the labour union is active and what is the mindset of the union leader. Is he a troublemaker? Please think it over and choose the right option.
Thanks,
Dinesh Divekar