In the present study, we investigated the physiological effects of smartphone use at night when the display luminance and white balance were differently manipulated. Two levels of luminance and two types of white balance were combined to form four types of displays. Subjects were instructed to use smartphones between 23:00 to 01:00 twice a week for two weeks, and for each trial, subjects were given one of the four display types. Melatonin concentration in the saliva, body temperature and heart rate were measured before and after each experiment. The experimental result showed that the low luminance display supported melatonin secretion and thermoregulation compared to the high luminance display. With regard to the white balance, higher melatonin level was observed when using the display that filtered blue light. The low luminance display together with yellowish tint best supported restful sleep at night in terms of every physiological response. This study collectively demonstrates that bright and blue light emitted from smartphone displays adversely affect melatonin secretion, body temperature, and heart rate, and therefore, suggests the use of a display with low luminance or a display that filters blue light for a restful sleep at night.

1. Introduction
 2. Method
  2.1. Subjects
  2.2. Stimuli
  2.3. Procedure
  2.4. Measurement of melatonin in the saliva
 3. Results
 4. Discussion
 5. Conclusion
 REFERENCES

• Nooree Na(Samsung Electronics)
• Hojun Choi(Department of Bio and Brain Engineering, KAIST)
• Kyeong Ah Jeong(Department of Industrial Design, KAIST)
• Kyungah Choi(Department of Industrial Design, KAIST)
• Kyungsun Choi(Department of Bio and Brain Engineering, KAIST)
• Chulhee Choi(Department of Bio and Brain Engineering, KAIST)
• Hyeon-Jeong Suk(Department of Industrial Design, KAIST) Corresponding author