The light exposure environment has changed considerably in recent years. It is common for people to spend less time outdoors and therefore have reduced exposure to daylight. In addition, the use of electronic devices, such as smartphones, tablets, and computers, has increased the light exposure at night. Previous studies reported that an inappropriate light exposure environment is linked to negative health effects in humans. As bipolar disorder is associated with light exposure, including bright light therapy, dark therapy, and light hypersensitivity, light exposure in daily life may affect the medical conditions of bipolar disorder. However, to the best of our knowledge, no study has investigated the association between the light exposure environment in daily life and the pathophysiology of bipolar disorder. Therefore, we objectively evaluated the daily light exposure of 218 outpatients with bipolar disorder, and analyzed the association between light exposure and their medical conditions. We found that light exposure is significantly associated with depressive and manic symptoms, sleep quality, and circadian rhythm in patients with bipolar disorder. Further studies are necessary to establish the association between light exposure and recurrence of mood episodes.
 Author's abstract