This study will investigate the potential benefit of scheduled natural daylight exposure to improve glucose control in type 2 diabetes (T2D) patients and to unravel molecular mechanisms underlying the effects of natural daylight on circadian clocks and (glucose) metabolism in human skeletal muscle from T2D patients. Participants will stay at our research facilities and will be exposed to natural daylight or artificial light during the daytime over 4.5 days in a randomized cross-over design. For both conditions, the evening and night will be spent under standardized dim and dark conditions.

Obesity and type 2 diabetes (T2D) are both strongly associated with a westernized lifestyle of low physical activity levels and high caloric intake. However, recently it has been recognized that also our 24-hour culture, characterized by working and eating late, reduced sleep (quantity and quality), and excessive light exposure in the evening and at night, should be considered as lifestyle factors that may negatively impact metabolic health. In this context, a factor that is often overlooked and underestimated is the lack of natural daylight since most people spend almost their entire work time in indoor office environments with limited access to natural daylight through windows.

Therefore, this study investigates the potential benefit of scheduled natural daylight exposure to improve glucose control in T2D individuals and to unravel molecular mechanisms underlying effects of natural daylight on circadian clocks and (glucose) metabolism in human skeletal muscle from T2D patients. For this purpose, overweight to obese T2D patients (male and female) will undergo a randomized cross-over trial in which each subject serves as its own control. Participants will stay at our research facilities and will be exposed to natural daylight or artificial light during the daytime over 4.5 days. For both conditions, the evening and night will be spent under standardized dim and dark conditions, respectively. Over these two 4.5 days intervention periods, extensive 24h metabolic profiling will be conducted, including 24h continuous glucose monitoring, 24h blood and saliva sampling, 24h blood pressure and heart rate assessments, 24h core and skin temperature assessments, 24h energy expenditure and whole-body substrate metabolism, and a skeletal muscle biopsy and a mixed meal tolerance test with indirect calorimetry will be performed on the respectively last test day.

Inclusion Criteria:

• Participants are able to provide signed and dates written consent prior to any study specific procedures
• Male + females (postmenopausal defined as at least 1 year post cessation of menses)
• T2DM duration at least 1 year
• BMI: ≥ 25 kg/m²
• Age: 40-75
• Well-controlled diabetes with respect to glycemic control and on stable anti-diabetes medication regimes
• Habitual bedtime of 23:00 ± 2h
• Regular sleep duration (7-9 h/night)
• Stable dietary habits: no weight gain or loss > 5 kg in the last three months

Exclusion Criteria:

• Insulin treatment
• Uncontrolled hypertension
• Signs of active diabetes-related co-morbidities like active cardiovascular diseases, active diabetic foot, polyneuropathy or retinopathy
• Signs of active liver or kidney malfunction
• Use of SGLT2 inhibitors
• Using > 400mg caffeine daily (more than 4 coffee or energy drink)
• Extreme early bird or extreme night person (score ≤30 or ≥70 on MEQ-SA questionnaire)
• shift work or travel across more than one time zone in the 3 months before the study
• Heavily varying sleep-wake rhythm
• Frequent engagement in programmed exercise as judged by the investigator
• Any medication that will interfere with the study outcomes or hamper the safety of the participant
• Alcohol consumption of >2 servings per day for men and >1 serving per day for woman
• Significant food allergies/intolerance (seriously hampering study meals)
• Subjects who do not want to be informed about unexpected medical findings during the screening /study, or do not wish that their physician is informed
• Previous enrolment in a clinical study with an investigational product during the last 3 months or as judged by the Investigator
• Smoking in the past 6 months