Some companies in Nigeria are forcing their employees to be doing overtime work against the employee’s wishes, especially the Chinese and Indian Companies in Nigeria. It was learnt that LORNA ENTERPRISES NIGERIA LIMITED located at Ikeja gave quarry letters to their employees who failed to show up for Saturday 18-01-2020 overtime work. Here is the explanation on the law of the land on this issue:
OVERTIME WORK: THE BASIS, THE NIGERIAN LABOUR LAW, COMPUTATION, AND MODE OF PAYMENT.
Legislative actions, practices, customs, Treaties, and Conventions have standardized working hours as eight ( hours per day and five (5) days per week. The Treaty of Versailles of June 28, 1919, which officially established the International Labour Office, now known as The International Labour Organization (ILO), puts a peg of finality on work hours and affirmed eight hours work day and this was subsequently ratified by its 52 Nations.
Nigeria is a signatory to the International Labour Organization and by implication to its treaties and conventions. It is right to posit therefore that the official work hour in Nigeria is eight hour, five days a week.
However, it is important to note the following clauses in the Nigerian Labour Act 1990:
Section 13, subsection 1, which states that Normal hours of work in any undertaking shall be those fixed:
1. by mutual agreement; or
2. by collective bargaining within the organization or industry concerned; or
3. by an industrial wages board
The implication of this pronouncement by the Act is that the responsibility for the determination of hours of work rests squarely on agreement between the employer and the employee (Contract or mutual agreement), or by collective bargaining (Employers Associations and Labour Unions or workers representatives) or an industrial wages board where there is no instrument for collective bargaining.
However, the National Minimum Wage (Amendment) Act, 2011, adopted by the ILO on 15th March 2011 states clearly that normal full-time work hours in Nigeria shall be forty hours per week.
Section 9 of the National Minimum Wage Act further justifies the above assertion when it defines wage to means total emolument paid every month to a worker, for performing services on the basis of forty-hour week, howsoever the emolument may be designated.

WHAT IS OVERTIME WORK?
Simply define, overtime is the amount of time a worker worked in excess of the standard working hour as defined by law, practice or customs or collective agreement. Overtime work is mostly regulated by the contract of employment and standard employment practices.
Section 13, subsection (2) of the Nigerian Labour Acts 1990, defines overtime as Hours which a worker is required to work in excess of the normal hours fixed under subsection (1) of this section shall constitute overtime.

JUSTIFICATION FOR OVERTIME WORK AND PAYMENT
It is important to states right from the onset that overtime work is not compulsory but it is permitted by law and most organizations. The peculiarity of company’s operations may sometimes demand that employees put in some extra hours at ensuring that their works are completed and daily deliverables met.

Legally I don't have a clue.But technically I think sometimes companies do have peak periods when the workloads are urgent and excess standard timeline, it is only natural for the comany to expect the workers to compile base on mutually agreed terms of engagement.