Summer Heat and Sleep: What Warm Nights Do to Rest

Last updated: October 2026

Warm nights cost sleep: in survey data from 765,000 US adults, warmer nighttime temperatures were linked to more nights of insufficient sleep, with the largest effect in summer (Obradovich et al., 2017). A systematic review found higher outdoor and indoor temperatures linked to poorer and shorter sleep, with no solid evidence that people quickly adapt (Chevance et al., 2024). Below is what heat does to sleep, who is most affected, and what the pillow does for the neck whatever the temperature.

Key takeaways

In data from 765,000 US adults, warmer nights were linked to more nights of insufficient sleep, most in summer (Obradovich et al., 2017).
A systematic review linked higher temperatures to poorer, shorter sleep and found no solid evidence of quick adaptation (Chevance et al., 2024).
A Pillowise fitting takes three measurements (shoulder width, neck circumference and neck length) and adds sleeping position and bed type to choose from seven sizes.

Does heat affect sleep?

A review of the thermal environment and sleep found that heat exposure increases time awake and reduces both slow wave sleep and REM sleep, and that humid heat makes these effects worse (Okamoto-Mizuno & Mizuno, 2012).

The body cools slightly as sleep begins. A warm, humid room works against that.

What do large studies show about warm nights?

An analysis of survey responses from 765,000 US adults between 2002 and 2011 compared reported sleep with local nighttime temperatures (Obradovich et al., 2017).

Warmer nights were linked to more nights of insufficient sleep. The study used outdoor temperatures and self-reported sleep, so it shows a link at the population level rather than what happens in any one bedroom.

Who is most affected?

In the US data, the effect was largest in summer and among older and lower-income respondents (Obradovich et al., 2017).

A systematic review of ambient heat and sleep found the link strongest in the warmest months, in vulnerable groups, and in urban areas, and found no solid evidence that people adapt quickly to sustained heat (Chevance et al., 2024).

What does pillow height change overnight?

Heat and the neck are separate questions, and the pillow belongs to the second one. In a radiographic study of 16 adults, raising the pillow from 0 to 10 to 20 cm increased T1 slope and C2-7 Cobb angle while neck tilt decreased (Kim et al., 2015).

In a pilot EMG study of 15 adults lying on their side, the height matched to shoulder width produced the lowest neck and shoulder muscle activity (Jiao et al., 2025).

Why do generic pillows miss?

A standard contoured pillow, not fitted to the individual, performed no better than ordinary shapes in a randomized study of 106 side sleepers (Gordon et al., 2009). No single optimal pillow height can be specified, because it depends on four variables that differ from person to person: cervical alignment, body dimension, contact pressure, and muscle activity (Lei et al., 2021).

What we measure

Pillowise is a fitted cervical pillow that licensed healthcare providers measure and dispense, made in seven sizes.

Our sleep specialists take three measurements: shoulder width, neck circumference, and neck length. Combined with sleeping position and bed type, an algorithm identifies the best-fitting pillow from seven sizes.

Shoulder width is the measurement the EMG study found mattered most. A height matched to it produced the lowest neck and shoulder muscle activity during side sleeping (Jiao et al., 2025), and body dimension is what drives the individual optimum (Tian et al., 2025).

A systematic review of 11 studies covering 309 participants identified a 7 to 11 cm unloaded central height among the parameters supporting spinal alignment and sleep comfort (Radwan et al., 2021). The size is chosen from those three measurements plus sleeping position and bed type, rather than from a single stock height.

Pillowise pillows are made from premium Dutch memory foam and certified to OEKO-TEX Standard 100, meaning every component has been tested against the standard's limits for harmful substances.

Scope

The heat studies measured temperature and sleep, mostly through self-report and outdoor readings, not pillows. No study has tested whether a pillow changes sleep in hot weather, and we make no claim about pillow temperature. The pillow studies are small, and the most recent systematic review found no clear advantage for any pillow type in chronic neck pain (Ghosh et al., 2025).

For providers

Summer is a natural time to ask patients whether heat is costing them sleep, especially older patients, who were among the most affected in the US data.

Patients who sleep poorly in the heat often shift position more and sleep on top of the covers. Sleep position and bed type are still the inputs that matter for pillow height.

A fitted pillow extends the work of the visit into the hours between visits, alongside the exercise, manual therapy, and other care the provider is already delivering.

Common questions

Why is it harder to sleep when it is hot?

Heat increases time awake and reduces slow wave and REM sleep, and humid heat makes it worse (Okamoto-Mizuno & Mizuno, 2012). In population data, warmer nights were linked to more nights of insufficient sleep (Obradovich et al., 2017).

Do people get used to sleeping in the heat?

Not quickly, on current evidence. A systematic review of ambient heat and sleep found no solid evidence of rapid adaptation (Chevance et al., 2024).

Does a cooling pillow help in hot weather?

No study cited here has tested that, and we make no claim about pillow temperature. Pillowise is chosen for height, from three measurements plus sleeping position and bed type.

Is a contoured pillow from a store enough?

Not reliably. A standard contoured pillow, not fitted to the individual, performed no better than ordinary shapes in a randomized study of 106 side sleepers (Gordon et al., 2009), and no single optimal height can be specified because it depends on four variables that differ from person to person (Lei et al., 2021). A contour helps only if its height matches the person using it. That is what the fitting settles, in a provider's office.

Find a Pillowise provider or become one

Pillowise pillows are clinically measured and dispensed exclusively through licensed healthcare providers as part of broader sleep, posture, and recovery education.

To locate a Pillowise provider near you, visit: Find a Reseller

Licensed healthcare providers interested in offering Pillowise may register here: Become a Reseller

References

Chevance, G., Minor, K., Vielma, C., Campi, E., O'Callaghan-Gordo, C., Basagaña, X., Ballester, J., & Bernard, P. (2024). A systematic review of ambient heat and sleep in a warming climate. Sleep Medicine Reviews, 75, 101915. https://doi.org/10.1016/j.smrv.2024.101915

Ghosh, S. K., Goyal, M., & Goyal, K. S. (2025). Effect of pillow on pain, disability and sleep quality in patients with chronic neck pain: A systematic review. Rehabilitación, 59(3), 100922. https://doi.org/10.1016/j.rh.2025.100922

Gordon, S. J., Grimmer-Somers, K., & Trott, P. (2009). Pillow use: The behaviour of cervical pain, sleep quality and pillow comfort in side sleepers. Manual Therapy, 14(6), 671-678. https://doi.org/10.1016/j.math.2009.02.006

Jiao, R., Xiao, W., Wang, M., Yu, S., & Li, H. (2025). The impact of pillow height on neck muscle activity: A pilot study. Sleep and Breathing, 29(1), 40. https://doi.org/10.1007/s11325-024-03219-6

Kim, H. C., Jun, H. S., Kim, J. H., Ahn, J. H., Chang, I. B., Song, J. H., & Oh, J. K. (2015). The effect of different pillow heights on the parameters of cervicothoracic spine segments. Korean Journal of Spine, 12(3), 135-138. https://doi.org/10.14245/kjs.2015.12.3.135

Lei, J.-X., Yang, P.-F., Yang, A.-L., Gong, Y.-F., Shang, P., & Yuan, X.-C. (2021). Ergonomic consideration in pillow height determinants and evaluation. Healthcare, 9(10), 1333. https://doi.org/10.3390/healthcare9101333

Obradovich, N., Migliorini, R., Mednick, S. C., & Fowler, J. H. (2017). Nighttime temperature and human sleep loss in a changing climate. Science Advances, 3(5), e1601555. https://doi.org/10.1126/sciadv.1601555

Okamoto-Mizuno, K., & Mizuno, K. (2012). Effects of thermal environment on sleep and circadian rhythm. Journal of Physiological Anthropology, 31(1), 14. https://doi.org/10.1186/1880-6805-31-14

Radwan, A., Ashton, N., Gates, T., Kilmer, A., & VanFleet, M. (2021). Effect of different pillow designs on promoting sleep comfort, quality, & spinal alignment: A systematic review. European Journal of Integrative Medicine, 42, 101269. https://doi.org/10.1016/j.eujim.2020.101269

Tian, S., Yao, C., Wang, Y., Cao, X., Sun, Y., Wang, L., & Fan, Y. (2025). The individualized optimal pillow height and neck support design for side sleepers. Medical and Biological Engineering and Computing, 63(2), 535-544. https://doi.org/10.1007/s11517-024-03204-x