Flextime programs have never been more popular than they are today. Google allows many employees to set their own hours. At Microsoft, many employees can choose when to start their day, as long as it’s between 9am and 11am. At the “Big Four” auditing firm KPMG, some 70 percent of employees work flexible hours.

Employees love these programs because they help them avoid compromises between home and at work. Yes, there are often boundaries within which a work day must begin and end, and at least some chunk of core hours that remain common across employees. But within those constraints, workers can schedule their office hours around the various other demands on their time, giving them greater control over their lives and allowing them to accomplish more. And because employees love the programs, companies have learned to love them, too. Research shows that in general, flexible work practices lead to increased productivity, higher job satisfaction, and decreased turnover intentions.

Yet the question lingers of whether employees who take advantage of flexible work policies incur career penalties for doing so. As noted in a recent paper by Lisa Leslie and colleagues, the evidence is mixed. Their research explored a potential reason for the widely varying outcomes: managers might look upon flextime favorably when they perceive a worker is using it to achieve higher productivity, and unfavorably when they perceive it being used to accommodate personal-life demands. Leslie et al. make the case that depending on what the manager attributes the flextime use to, the employee may be either rewarded or penalized.

We looked at another possible explanation for why some flextime-using employees and not others would experience negative career outcomes. Perhaps, we hypothesized, it matters in which direction an employee shifts hours. People seem to have a tendency to celebrate early-risers. Witness the enduring popularity of aphorisms like Ben Franklin’s “early to bed, early to rise, makes a man healthy, wealthy, and wise” or, in China, “a day’s planning should be done in the morning.” In the eyes of managers with power over careers, are employees who choose later start times stereotyped as less conscientious, and given poorer performance evaluations on average? Do the “larks” on a team hold a hidden edge over the “owls”?

We began our research by testing whether such a stereotype actually exists. We designed a laboratory experiment to discover the degree to which people made a natural implicit (that is, nonconscious) connection between words associated with morning (such as “sunrise”) or evening (such as “sunset”) and words associated with conscientiousness (such as “industriousness”). Across 120 participants, we found that on average people do make a greater natural implicit association between morning and conscientiousness.

With the general stereotype established, we went on to explore its impact in actual work settings, and on ratings provided by real supervisors. The field study we conducted tested the hypothesis that supervisor ratings of conscientiousness and performance would be associated with the timing of an employee’s work day. The hypothesis was supported. Across 149 employee-supervisor dyads, even after statistically controlling for total work hours, employees who started work earlier in the day were rated by their supervisors as more conscientious, and thus received higher performance ratings.

We conducted another laboratory experiment to test the same hypotheses in a more tightly controlled setting. We put participants in the role of being a supervisor, and asked them to rate the performance of a fictitious employee. We gave a performance profile to the supervisors, which was constant across everyone. However, in the “morning” condition we indicated that the fictional employee tended to work from 7am to 3pm, and in the “evening” condition we indicated that the fictional employee tended to work from 11am to 7pm. Everything else about the fictional employee and performance profile was identical across the conditions. Across 141 participants, we found that the research participants gave higher ratings of conscientiousness and performance to the 7am-3pm employees than to the 11am-7pm employees.

Thus, in three separate studies, we found evidence of a natural stereotype at work: Compared to people who choose to work earlier in the day, people who choose to work later in the day are implicitly assumed to be less conscientious and less effective in their jobs. But an additional finding must also be noted. In both the field study and the lab experiment, the effects were strongest for employees who had supervisors who were larks, and disappeared for employees who had supervisors who were night owls. (For those interested in further detail on the studies, our formal paper will be published later this year in the Journal of Applied Psychology.)

Of course, the implications of this research are not pretty. It seems likely that some employees are experiencing a decrement in their performance ratings that is not based on anything having to do with their actual performance. Organizations may be inadvertently punishing the employees who use flextime to start and finish working later in the day. And as accumulated poor performance ratings have detrimental effects on career advancement, this could partly explain why we often see flextime utilization having negative effects on employee careers.

The important implication is that senior managers must intervene in some way to keep supervisors from essentially punishing employees for using the very flextime policies their organizations endorse. Rather, they should be doing the opposite; if they encourage the use of flextime, they will produce the benefits noted by previous research. As with other areas of unintentional but proven bias, the advice is to increase managers’ awareness of their tendency to stereotype and why it is invalid. They must be continually reminded to recognize their cognitive tendencies and adjust for them. Managers must be especially diligent in rating the performance of employees based on objective standards, and not allowing implicit prejudices – such as their morning bias – to color their assessments.

Meanwhile, what is the individual employee to do? One message workers could take from this research is that, if they have the opportunity to use flextime, they might be better served by using it to move their schedules early in the day rather than later in the day. However, we would hesitate to recommend this, since a trend in that direction can only heighten the penalties for their colleagues whose lives outside work make the earlier hours difficult. More productively, they can raise the subject of hours and timing with their supervisors, and help make explicit the understanding that start time is immaterial.

One way or another, team leaders must come to accept that the people who use flextime to start their day late are not necessarily lazier than their early-bird colleagues. Otherwise, flextime policies that could serve both employees and employers well will become known, and avoided, as routes to dead-end careers.