Restless, disrupted, or just too-little sleep is a frustrating problem many people have, particularly on hot nights. But just how hot is too hot?

New research on a group of 50 volunteers aged over 60 and living in Boston, US, has found the optimal temperature range for the most restful sleep for older adults is between 20 to 25 °C (68 to 77 °F).

When temperatures climbed from 25 °C to 30 °C, the study's participants' sleep efficiency – the amount of time they spend asleep after crawling into bed for some shut-eye – dropped by as much as 10 percent.

That's no trivial amount. Previous studies have shown a 10 percent drop in sleep efficiency is enough to impair brain performance, increase stress, anxiety and fatigue, and affect the body's blood sugar level control the next day.

With nights getting hotter as the planet warms, the study's findings support measures to improve the thermal comfort of dwellings, especially aged care homes and public housing. Other research is finding solutions beyond air conditioning, like reflective paints and other building materials.

"As we grapple with the broader implications of climate change, we must not overlook its potential impact on something as fundamental as sleep," says Amir Baniassadi, an engineer and health researcher at Harvard Medical School, who led the study.

We already know poor sleep can have lasting impacts on our physical and mental health; it even changes the way we see people, affecting our relationships.

Historical data and longitudinal studies also show past temperature increases have likely impacted sleep patterns in meaningful ways, with further warming set to disrupt more sleepers.

By 2099, hotter temperatures may erode around 50 hours of sleep per person, per year. Elderly people are among the most vulnerable to heat stress, day or night.

Many sleep studies have been conducted in temperature-controlled lab settings, whereas this new study tracked people's sleep patterns and temperatures inside their own homes.

Participants, aged 65 years and over, had indoor air temperature and humidity sensors fitted in their bedrooms and wore a ring-like device overnight to track their sleep, skin temperature, heart rate, and movement.

In total, the researchers collected nearly 11,000 person-nights of sleep and environmental data to analyze.

While temperatures between 20 and 25 °C promoted the most restful sleep, the study found a "great deal of difference between people, meaning that each person has their own optimal temperature range for sleep, which may even change over time," says Baniassadi.

He suggests making small adjustments to your own personal sleep environment to optimize sleep, such as improving airflow and opting for lightweight sleepwear. Improving the thermal comfort of the building itself could make a bigger difference, though it's more difficult and costly.

Participants in this study lived in a variety of dwellings, from small government-subsidized apartments to private single-family homes, although most reported fairly high standards of living.

"Our study underscores the potential impact of climate change on sleep quality in older adults," Baniassadi and his colleagues write in their paper, "particularly those with lower socioeconomic status."

Likewise, people living in social housing and renters who often live in older buildings that are poorly insulated from the heat and cold also need adjustments to their homes to stay comfortable as temperatures increase day and night in cities around the world.

This includes system-wide policies to lift building standards for new builds, better insulate existing dwellings, and subsidies to encourage the uptake of solar panels to give everyone access to cheap electricity.

"In the face of climate change, these actions are not merely proactive measures, but necessary adaptations for our health and well-being," Baniassadi says.

The study has been published in Science of The Total Environment.