Hi,
You want to know whether it is legally right to engage an employee in a night shift for continuous 6 months.
Let's keep aside the legality part.
If an employee continuously works the night shift, there is a higher risk of the employee developing metabolic syndrome, which might lead to cardiovascular disease, insulin resistance, and neurological complications.
So, considering the employee's health (which is the primary wealth for the employee), it is suggested to rotate the night shift at frequent intervals.
Note: To the best of my knowledge, there is no restriction for keeping an employee on a continuous night shift, but please don't follow it wherever possible.