It could be that the sunlight exposure during the day generates more melatonin at night, resulting in better sleep. However as other commenters have pointed out

It could be that the sunlight exposure during the day generates more melatonin at night, resulting in better sleep. However as other commenters have pointed out, it's unclear how well controlled this study was. I would prefer to see a study where they moved workers around the office and compared the amount of sleep to sunlight exposure for each worker.