The restricted period is the block of overnight hours, usually 10pm to 6am, during which young workers (over school leaving age but under 18) generally can’t be required to work.
There are limited exceptions, such as certain hospital work, but only where no adult worker is available to do the job instead.
In practice
A 17-year-old warehouse workerA legal category that sits between employee and self-employed. Workers are entitled to certain rights including national minimum wage, paid holiday, and protection from unlawful deduction from wages, ... is asked to cover a 10pm-to-6am shift. Unless a specific exception applies and no adult worker is available, this falls inside the restricted period and shouldn't be required of them.
See also