(for KL Siew's Use Only)
First of all, I think your split shifts working hours: 8am-12am and 7pm - 11pm, are not in order. Although you may think they only work 8 hours per day, but under the Employment Act, you may have to pay overtime. This may be something you don't know. I would strongly suggest that you pay a personal visit to the local Labour Department and discuss the issue before you start talking about how to penalize them.