The eight-hour work day is based on employees sticking to a 40-hour, overtime-free workweek. Federal law doesn't specify a maximum numbers of hours an employee is allowed to work per day. State laws follow with a lack of policy specifics for adult workers. The only laws in place limiting work hours focus on children ages 15 or younger.
TL;DR (Too Long; Didn't Read)
Unless a worker is 15 or under, federal and state laws do not limit the number of hours in a work day. However, union agreements or corporate policy may step to keep working hours in check.
The Federal Fair Labor Standards Act of 1938 dictates policy for most workers. According to an interpretation of the FLSA by the U.S. Department of Labor's the act does not limit the number of hours in a day or days in a week an employee must work, including overtime hours, if the employee is at least 16 years old. Any time worked over 40 hours per week, however, is considered overtime. According to the FLSA, employees must be paid a minimum of time and one-half for any additional hours worked.
A few states have rules that help limit the hours most employees work, but none are absolute. Even in states with laws that treat the issue, the limits on hours worked are indirect and weak. In California, for example, an employee cannot be fired or disciplined for refusing to work more than 72 hours in a given week. New York labor labor requires employers to pay employees working more than 10 hours a day one extra hour of pay, but at the minimum legal wage. Otherwise, as one labor lawyer commented, no one may work more than 168 hours per week, but only because physics prevents it -- there are only 168 hours in a week, after all.
Laws for Younger Workers
The FLSA provides that workers under 16 years of age cannot work over 8 hours a day, with a lower limit of 3 hours daily on school days. Some states have slightly less restrictive limits. Idaho, for instance, limits workers under 16 to nine hour working days, with no further reduction for school days.
Although the FLSA normally prevails over state laws, small businesses are exempt from its provisions. Further, if the employer is not engaged in interstate commerce, only the less restrictive state laws apply. In practice, this can be a somewhat tricky distinction. An office worker mailing to local customers is not covered, but an office worker preparing promotional material mailed to individuals in other states probably is.
Certain workers are exempt from the provisions of the FLSA and are covered by other laws and regulations limiting hours worked. Unionized workers are protected by labor agreements with employers. Other workers are covered by safety regulations. These include airline pilots, physicians, truck drivers and miners working underground.
- U.S. Department of Labor Wage and Hour Division: The Fair Labor Standards Act Of 1938, As Amended
- Nolo: Can My Boss Force Me to Work Overtime
- U.S. Department of Labor Wage and Hour Division: Fact Sheet #14: Coverage Under the Fair Labor Standards Act (FLSA)
- U.S. Department of Labor: Handy Reference Guide to the Fair Labor Standards Act
- U.S. Department of Labor Wage and Hour Division: Overtime Pay