Employers, in the U.S., are obligated to provide various religious accommodations to their employees, e.g. reasonable prayer breaks etc. But some religious pracices require types of abstinence, such as deprivation from eating and drinking all day, that take a toll on the person's executive functioning, attention, and overall cognition.
If, as a result of such a religious observance, an employee's work performance deteriorates, to which degree, if any, is the employer required to tolerate it, i.e. absorb business loss to be compliant with accommodation?