Most written contracts of employment stipulate the number of hours that an employee will be required to work over a given period, and when those hours must be worked. There are also likely to be contract terms covering rest breaks, including details of their length and whether they are paid or unpaid. An employer that operates a shift system is likely to include details of the system in employees' contracts, either directly in the contract itself or by incorporating into the contract a collective agreement or other document where the detail can be found.
Even if they do not have a written contract of employment, all employees must be given a written statement of their terms and conditions relating to hours of work, including any terms and conditions on normal working hours (documenting the contract).
Many employers operate contracts of employment that give them flexibility over working hours. It is common, for example, for employers to have the power to require employees to alter their working hours by changing the shift that they work, or to extend their working hours by working overtime.
Overtime can be of various types. It may be voluntary overtime, which the employee can choose whether or not to work. It may be guaranteed overtime, which the employer must provide. Or it may be compulsory overtime, which the employee must work if asked to do so by the employer but which may or may not be guaranteed. There may be conditions attached to an employer's power to require overtime working, such as a requirement that the employer should give the employee a certain number of hours' notice. Commonly, overtime is paid at a premium rate of pay or attracts the right to time off in lieu.
It is important to bear in mind that the flexibility that an employer has to regulate employees' working time may in practice be limited by implied terms in their employment contracts. For example, the courts have ruled that if an employer asks its employees to work such long hours that they become ill, it is breaching its implied duty to protect their health and safety, even if it has an express power in their contract to require them to work overtime.