300 participants are expected to take part in this Stanford-led study in Makassar, Indonesia, and participants are currently getting enrolled. They will help researchers understand how extreme heat inside the home affects sleep, heart health, physical activity, mental health, and quality of life.

Extreme heat does not stop at the front door. In homes without air conditioning, indoor temperature and humidity can remain high for hours, including the hours when people are trying to rest and sleep. The research team is studying what that exposure does to the body—and whether a relatively simple change to the home can help.

The project, conducted with Hasanuddin University in Makassar, Indonesia, will combine Fitbit measurements, indoor environmental sensors, and repeated surveys. Its central question is practical: can a reflective white roof make a home cooler, and can that change be detected in the health and daily lives of the people who live there?

The broader project also aims to recruit up to 300 households for its environmental, survey, and reflective-roof research.

Heat as a household health exposure

The study’s formal title is Interventions to Counter Extreme Heat: Development, Implementation, Evaluation, and Creating a Roadmap for Sustainability in Climate-Vulnerable Informal Settlement Communities. It is led by John Openshaw, MD, of Stanford University, with Whitney Weber, PhD, serving as the study’s central contact. Stanford University and Hasanuddin University are the participating institutions.

The team is focusing on coastal informal settlements in Makassar. These communities offer an important setting for studying heat inside the home: eligible households do not have air conditioning, and residents may have limited options for changing the structures in which they live.

The researchers are asking three connected questions:

  1. How does extreme heat affect health and wellbeing in climate-sensitive informal settlements?
  2. What physiological and mental-health effects are associated with extreme heat inside the home?
  3. Can reflective roofs reduce indoor temperatures and, in turn, lessen heat-related effects on health?

The study begins with a clear hypothesis. The researchers expect higher heat exposure inside the home to be associated with higher resting heart rate, poorer sleep, less physical activity, greater mental-health strain, and lower quality of life. They also expect accessible heat-mitigation measures—particularly reflective roofing—to lower indoor temperatures and improve at least some of those outcomes.

A roof intervention measured from several angles

A reflective roof is a straightforward idea: a light-colored surface reflects more solar energy than a darker roof, reducing the amount of heat absorbed by the building. But a cooler roof is not automatically the same thing as a meaningful health benefit. This study is designed to measure the full path from the physical intervention to the participant’s experience.

Environmental sensors will be placed in the main bedroom of participating homes, where they will continuously record indoor temperature and humidity. The sensors may remain in place for up to five years. Depending on the household’s assignment to the delayed or intermediate intervention group, the roof will be treated with reflective paint during the study.

That design allows the team to compare several layers of evidence:

  • whether the roof intervention changes indoor temperature and humidity;
  • whether changes in the home environment correspond with changes in sleep, heart measures, breathing, activity, or mobility;
  • whether participants report fewer heat-related symptoms or disruptions to daily life; and
  • how households already adapt to heat, and which additional approaches residents view as useful and sustainable.

The roof is therefore not being evaluated through temperature readings alone. Household conditions, self-reported experience, and wearable measurements will be considered together.

What participation involves

The planned wearable cohort includes adults over 18 who live in eligible households and can use a smartphone, install the required software, maintain a sufficient data plan, and keep both the phone and Fitbit charged. Participants are expected to spend roughly 95% of their nights at home so that the environmental measurements in the house can be meaningfully paired with their wearable and survey data.

Up to two people in a household may take part in the wearable portion of the study. Participants will be asked to wear a Fitbit during a six-month data-collection period each year, expected to run from September 1 through February 28. This window spans the hottest dry months as well as somewhat cooler, wetter months, giving the team a wider range of heat conditions to study.

During the first 14 days of each annual collection period, wearable participants will complete a brief daily symptom report. After that initial period, they will complete a survey every two weeks for the remainder of the six months. Those follow-up surveys cover health, symptoms, heat exposure, and travel away from the home. Participants may be encouraged to keep wearing the device outside the formal six-month window to make the routine easier to maintain, although study data may not be collected during those additional months.

The head of each household will complete a broader set of surveys at enrollment. Topics include household characteristics, health history, demographics, heat adaptation, exposure to heat and weather events, water and sanitation, physical activity, and quality of life. Additional household surveys are planned quarterly.

The overall project may continue for up to five years, creating the possibility of repeated observations across several annual heat seasons rather than a single hot week or month.

What the Fitbit data can reveal

Fitbit is being used as a measurement tool, not as the intervention itself. The planned data include activity, heart rate, location, profile, and sleep information. The research team is particularly interested in:

  • sleep duration and quality;
  • heart rate and heart rate variability;
  • steps, distance, active minutes, and broader mobility patterns;
  • breathing rate;
  • Activity Zone Minutes;
  • cardio fitness estimates such as VO2 max;
  • oxygen saturation;
  • temperature-related measurements; and
  • other measures of physiological stress and recovery available through the device.

Sleep quality is the study’s primary wearable outcome. Secondary outcomes include heart rate variability, mobility, temperature, activity, and breathing rate. The value of these measures comes from their timing: researchers can compare changes in the participant’s physiology and behavior with temperature and humidity recorded inside that participant’s home.

No single signal will answer the study’s questions on its own. A night of disrupted sleep may coincide with a hotter bedroom, but surveys can add context about symptoms, travel, illness, or other events. Likewise, a reduction in steps during an extreme-heat period may become more interpretable when considered alongside self-reported activity and quality of life.

Bringing field data into StudySync

Wearable participants will connect their Fitbit through a smartphone application. StudySync provides the data pathway that securely brings authorized Fitbit records from participants in Makassar to the research team’s server for analysis.

That link matters for a study with repeated measurement periods and participants living far from the Stanford research team. Instead of relying only on measurements collected during occasional site visits, the researchers can build a longitudinal record spanning daily life, changing seasons, and the period before and after a household receives the roof intervention.

StudySync is one part of a broader field protocol. The platform carries the wearable data; the environmental sensors establish what was happening inside the home; and the surveys capture symptoms, perceptions, behavior, and household context. The analysis will bring those sources together.

Four aims, one practical outcome

The protocol organizes the work into four aims:

  1. Characterize heat exposure. Continuously measure temperature and humidity inside homes in informal settlements.
  2. Quantify health effects. Pair self-reported outcomes with wearable measures of mobility, cardiac strain, stress, and sleep during extreme-heat exposure.
  3. Understand existing adaptation. Document how residents already respond to heat and which solutions are feasible in their communities.
  4. Evaluate reflective roofs through community co-design. Implement cool roofs with community participation, then assess changes using household sensors, questionnaires, and wearable data.

The end goal is not merely to show that reflective paint can alter a thermometer reading. It is to learn whether a feasible household intervention can improve sleep, reduce physiological strain, support daily activity, and protect wellbeing—and to develop a roadmap that communities facing similar heat exposure could realistically sustain.

Ethics, withdrawal, and future research

The application reports approval through an expedited review by the Stanford Human Subjects Review Board. Participation is voluntary, and participants may withdraw at any time by contacting the field team. The protocol also states that participants can access their data during the study or after withdrawal.

The planned study period runs through 2031, with data retention extending beyond the end of collection to support analysis. The team intends to publish the results, potentially through several papers covering methods, participation and compliance, and the eventual health findings. After relevant results have been published, the researchers plan to share depersonalized data, data dictionaries, and analysis code for other researchers.

Where the study stands

Participant enrollment is currently underway. The research team is working toward its end goal of 300 wearable participants across as many as 300 households. As enrollment and field collection progress, the study will begin assembling the paired record at the heart of the project: what the home felt like, what the participant experienced, what the wearable measured, and whether a cooler roof changed any of it.