The following information is from a publication by the Equal Employment Opportunity Commission (EEOC) at https://www.eeoc.gov/policy/docs/accommodation.html#modified
Must an employer allow an employee with a disability to work a modified or part-time schedule as a reasonable accommodation, absent undue hardship?
Yes. A modified schedule may involve adjusting arrival or departure times, providing periodic breaks, altering when certain functions are performed, allowing an employee to use accrued paid leave, or providing additional unpaid leave. An employer must provide a modified or part-time schedule when required as a reasonable accommodation, absent undue hardship, even if it does not provide such schedules for other employees.
Example A: An employee with HIV infection must take medication on a strict schedule. The medication causes extreme nausea about one hour after ingestion, and generally lasts about 45 minutes. The employee asks that he be allowed to take a daily 45-minute break when the nausea occurs. The employer must grant this request absent undue hardship.
For certain positions, the time during which an essential function is performed may be critical. This could affect whether an employer can grant a request to modify an employee's schedule. Employers should carefully assess whether modifying the hours could significantly disrupt their operations -- that is, cause undue hardship -- or whether the essential functions may be performed at different times with little or no impact on the operations or the ability of other employees to perform their jobs.
If modifying an employee's schedule poses an undue hardship, an employer must consider reassignment to a vacant position that would enable the employee to work during the hours requested.
Example B: A day care worker requests that she be allowed to change her hours from 7:00 a.m. - 3:00 p.m. to 10:00 a.m. - 6:00 p.m. because of her disability. The day care center is open from 7:00 a.m. - 7:00 p.m. and it will still have sufficient coverage at the beginning of the morning if it grants the change in hours. In this situation, the employer must provide the reasonable accommodation.
Example C: An employee works for a morning newspaper, operating the printing presses which run between 10 p.m. and 3 a.m. Due to her disability, she needs to work in the daytime. The essential function of her position, operating the printing presses, requires that she work at night because the newspaper cannot be printed during the daytime hours. Since the employer cannot modify her hours, it must consider whether it can reassign her to a different position.
Other issues include: