Sleeping in a warmer bedroom during the summer is associated with poorer sleep quality and feeling less focused and productive the next day, according to new research published in the Journal of Environmental Psychology. By combining bedroom air sensors, wearable sleep rings, and smartwatch check-ins over several weeks, the researchers found that night-to-night increases in indoor heat corresponded with more restless sleep and small dips in self-reported daytime productivity. As nighttime temperatures climb globally, the findings point to bedroom temperature as an everyday factor tied to nightly rest and daytime focus.

Perceived productivity refers to how focused and capable of completing tasks a person feels during daily activities. The authors draw on models of work recovery, which propose that people need adequate rest to replenish the attention and self-control they use up during the day. They reasoned that if warm bedrooms degrade sleep quality, meaning how continuous, deep, and restorative a night’s sleep is, people may start the morning with fewer mental resources.

Warm weather can erode nightly rest and strain daytime work output, though those problems have mostly been studied apart. Globally, a 2022 study of more than seven million wearable sleep recordings estimated that rising temperatures cost people roughly 44 hours of sleep per year, while a 2018 meta-analysis of workers exposed to occupational heat linked thermal strain to worse health and lower job productivity. Direct tests of bedroom heat, sleep, and next-day thinking have largely used small climate chambers rather than real homes.

Less was known about how night-to-night shifts in a person’s bedroom microclimate, meaning the temperature, humidity, and air quality in their sleeping space, relate to sleep quality and perceived productivity the next day. The authors also tested whether the autonomic nervous system, the nerve network that automatically regulates involuntary functions like heart rate, relates to hot nights, sleep, and focus. Christian Rominger of the University of Hildesheim in Germany and colleagues at the University of Graz and the Technical University of Vienna tracked adults at home in summer.

After excluding volunteers with sleep or cardiovascular disorders, the team analyzed data from 82 adults in Austria aged 20 to 65 (average age 28), including 56 women, 24 men, and 2 gender-diverse participants. Most worked in offices, and 33 had a cooling system at home. One participant lacked smartwatch survey data, and three lacked daytime heart rate recordings.

Data collection ran in July and August 2025 in Vienna, Graz, and nearby suburbs, where outdoor temperatures ranged from about 10 to 37 degrees Celsius (50 to 99 degrees Fahrenheit). Participants were tracked for two to three weeks within a larger Open Science Framework project that included an intervention, which had no statistically significant effects on the variables studied here and was accounted for in statistical checks. Removing sick days or nights away from home left 1,189 paired nights and following days.

Participants placed an air monitor near bed height, away from sunlit windowsills, to log bedroom temperature, relative humidity, and carbon dioxide every five seconds during sleep. Nighttime bedroom temperatures averaged 24.0 degrees Celsius (75.1 degrees Fahrenheit), spanning 18.5 to 28.7 degrees Celsius (65.4 to 83.7 degrees Fahrenheit).

Participants also wore an Oura Ring, which tracks finger blood flow, skin temperature, oxygen, and movement to calculate an objective sleep score from 0 to 100 (averaging about 79, with scores typically varying by about 4.6 points). The ring estimated sleep efficiency (the share of time in bed spent asleep), time to fall asleep, restless periods, deep sleep (the restorative slow-wave stage), and rapid eye movement (REM) sleep (tied to dreaming and memory). Participants rated their sleep each morning and completed a sleep questionnaire after week one.

During the day, an Apple Watch recorded heart rate and steps while sending eight prompts between 7:00 a.m. and 9:00 p.m. Using ecological momentary assessment, which asks how people feel in real time during normal routines rather than relying on memory, each prompt asked how focused and productive participants felt on a five-point scale (averaging about 3.1, varying by about 0.8 points). Participants also rated daytime sleepiness and mood, letting the team test how nights warmer than a person’s usual bedroom average related to sleep and next-day focus.

Daily wearable sleep scores tracked with morning self-ratings and inversely with next-day sleepiness, while their link to the one-time sleep questionnaire was not statistically significant. When a person’s bedroom was warmer than their own usual average, their objective sleep score showed a small drop of about half a point on the 100-point scale per additional degree Celsius. Warmer nights also went along with worse morning sleep ratings and greater next-day sleepiness.

Among individual sleep metrics, warmer bedroom nights corresponded with more restless periods and shorter REM sleep. A tendency toward shorter deep sleep on warmer nights was not statistically significant, and light sleep, time in bed, sleep efficiency, and time to fall asleep had no statistically significant links to bedroom heat.

Sleeping in a bedroom warmer than one’s personal average predicted slightly lower self-reported focus and productivity the next day, about 0.04 points on the five-point scale for each additional degree Celsius. That dip held up after adjusting for weekends, prior-day productivity, home cooling systems, and next-day mood, suggesting that people were not just rating themselves as less focused because heat soured their mood. Average bedroom temperatures across households showed no statistically significant links to overall sleep or productivity, pointing instead to warmer-than-usual nights within a person’s own room.

Higher overnight objective sleep scores predicted higher perceived productivity the next day, even after accounting for mood and household factors. Among specific sleep stages, longer deep sleep emerged as the primary metric positively tied to next-day focus, whereas bedroom temperature’s link to deep sleep was not statistically significant. Because each link was tested in a separate analysis, the study does not show that poorer sleep is the route through which warm bedrooms relate to next-day productivity.

Day-to-day shifts in bedroom humidity were not statistically significant for sleep quality or next-day productivity, though participants with higher average bedroom humidity reported slightly higher average productivity. Unexpectedly, nights with higher bedroom carbon dioxide, a marker of stale air, went with slightly higher objective sleep scores, while their day-to-day link with next-day productivity was not statistically significant.

That carbon dioxide sleep result contrasts with controlled ventilation research, such as a 2016 field experiment in student dormitory bedrooms, which found that lowering overnight carbon dioxide via open windows or fans improved objective sleep quality and next-day mental performance. The authors note that carbon dioxide stayed relatively low overall in their Austrian bedrooms (averaging 793 parts per million), and they suggest that unmeasured factors may explain the pattern. Closing windows, for example, raises carbon dioxide but also blocks outdoor noise that could otherwise disturb sleep.

Smartwatch data did not support daytime heart rate as a bridge between hot nights and less productive days, as its links with bedroom temperature and sleep quality were not statistically significant. Instead, higher daytime heart rate went with slightly higher perceived productivity, even after adjusting for step counts and mood. Rather than indicating heat strain, the authors suggest a faster heart rate may reflect heightened alertness and task engagement.

These day-to-day patterns align with an earlier review of research on heat and mental health, which linked high temperatures to worsened psychiatric symptoms, more aggression and self-harm, and impaired thinking, and which proposed poor sleep on hot nights as one possible contributor. Whereas that broader review examined clinical mental health, mood, and objective cognitive performance, the new study specifically evaluated wearable-tracked sleep and self-reported daily productivity.

There are a few things to keep in mind when interpreting these results. Because the study observed people naturally at home rather than assigning room temperatures, it cannot establish that bedroom heat directly caused the dips in sleep or focus. The day-to-day effects were also small, and the links with individual sleep stages were tested one by one, so the REM sleep result in particular should be treated cautiously. Productivity was measured via two self-report items rather than objective work output or cognitive tests, which the authors note do not always match subjective impressions after poor sleep.

The researchers lacked night-by-night records of bedroom habits, such as whether participants opened windows, ran a fan or air conditioner, or shared their bed with a partner, child, or pet. Data collection lasted only two to three weeks during a moderate summer period in mid-sized Austrian cities among a relatively young, mostly office-working sample. The authors note that mild heat and gradual bodily adaptation to summer warmth may have muted the effects compared with sudden early-summer heat waves or hotter regions.

Future studies could pair wearables and real-time check-ins with objective cognitive tests across seasons or years to track how heat adaptation shapes sleep and work output. Trials testing bedroom cooling strategies across broader groups could also test whether cooling sleeping spaces directly protects next-day productivity.

The study, “Rising temperatures – deteriorating productivity: An examination into day-to-day dynamics of temperature, sleep, and perceived productivity,” was authored by Christian Rominger, Martha Schneider, Gerald Schweiger, Katja Ceplak, Kristina Kirschner, Julia T. Aulehner, Ruth Höpler, Christian Peer, and Andreas R. Schwerdtfeger.