## 2 Materials and Methods
### Participants  
Thirteen participants and their informal caregivers (\(n=13\)) received information about the research project and were willing to participate. Written informed consent was obtained from all the participants and their caregivers. One couple decided to stop before data collection started, and 1 couple decided to stop on the 1 st week of data collection due to changes in their health conditions (pp06). Therefore, the total sample size of this study was 11 participants. One participant, pp11, completed two of the four phases and decided not to complete the other two phases due to sudden deterioration of the condition, which required full attention. We obtained complete data of four phases of 10 participants. The participants were recruited by formal caregivers of the Innovate Dementia network [37] and through posts via social media and Alzheimer Cafe meetings in the Netherlands within the period of September 2019 to June 2020. The study protocol was approved (23 April 2015) by the Institutional Review Board of the mental health care institute Eindhoven (GGzE) and by the Medical Ethics Committee (METC) of Noord-Brabant (29 August 2018, P1826), the Netherlands. Both participants and their informal caregivers signed a written informed consent form, in accordance with the Declaration of Helsinki (Seoul Revision, 2008) [38] and the General Data Protection Regulation (AVG; www.eugdpr.org) (as applied in 2018).  
The inclusion criteria for the study were: (1) having a primary diagnosis of dementia, diagnosed by a geriatric, neurologist, or psychiatrist, based on the criteria of the _Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition_ (DSM-V) [39]; (2) living in a home situation; (3) having sleeping problems based on the Neuroppsychiatric Inventory Questionnaire; (4) having sufficient cognitive ability to participate in this study, a score of 24 or above on the Mini-Mental State Exam (MMSE), and being mentally competent to decide for themselves to participate; (5) no visual disabilities or wheelchair dependence; and (6) having an actively involved informal caregiver who frequently checks the well-being of the participant. Patients were excluded from the study if any other neurological disorders or a serious eye disease, such as retinitis pigmentosa, was diagnosed, which makes light intervention incompatible.  
As Table 1 shows, the study population included 11 participants (5 women and 6 men) with a mean age of 78.1 years. The mean score on the Mini-Mental State Exam (MMSE), a questionnaire to measure cognitive impairment, was 24.6. A score below 24 can indicate mild to severe cognitive impairment and possible mental incompetence to participate in the study. For 8 participants, the informal caregiver was their spouse, while for the other 3 participants, it was their daughter (in-law). The mean age of the informal caregivers was 68.2 years. Eight participants were diagnosed with Alzheimer's dementia, 2 participants with Lewy body dementia, and 1 participant with frontotemporal dementia. All the participants experienced sleeping problems.  
\begin{table}
\begin{tabular}{l c c} \hline Gender & Participants & Caregivers \\ Male & 6 & 3 \\ Female & 5 & 8 \\ \hline Dementia type & & \\ Alzheimer’s & 8 & \\ Lewy body & 2 & \\ Frontotemporal & 1 & \\ \hline Relationship with participant & & \\ Spouse & & 8 \\ Daughter (in-law) & & 3 \\ \hline Mean age (SD) & 78.1 (8.56) & 68.2 (11.59) \\ \hline Mean MMSE score (SD) & 24.55 (2.3) & \\ \hline \end{tabular}
\end{table}
Table 1: Participant and caregiver demographic variables at baseline (\(n=11\)).