The main objective of the study is to improve subjective sleep quality using a multimodal intervention with an intervention and a control group. The multimodal intervention aims to improve circadian Zeitgeber strength. This will be achieved by optimizing daily light exposure, sleep, physical activity, and mealtimes.

The study lasts for 14 months and contains 6 visits. After the screening visit at the Centre for Chronobiology, Basel, Switzerland, participants are assigned to either the intervention or the control group which is followed by a 7-day baseline week at home and a co-design meeting to provide instructions to the intervention and the control group. At the occasion of 2 visits (spread over 6 months) at the participants' homes, different measurements will be performed such as saliva collection, questionnaires, cognitive performance tests, light and rest-activity measures. After the last visit at home, participants come for the second blood sample at the study centre. A follow-up visit 6 months after completion of the last data collection will provide qualitative feedback by the participants.

Inclusion Criteria:

at least 65 years old women and men

• Pittsburgh Sleep Quality Index (PSQI) > 5 (indicative of sleep problems)
• Living at home (including self-serviced retirement apartment)

Exclusion Criteria:

• Total blindness or VA < 0.5
• Progressive NCDs (e.g., cancer)
• Acute infections (e.g., Covid-19)
• Neurodegenerative disease
• Psychiatric disease
• Sleep disorder (sleep apnea, narcolepsy)
• Not German speaking

Participants are unaware whether they are in the intervention or the control group