How to sleep well in heat

Article written by Dr Greg Potter ( PhD, MSc, BSc)

Greg Potter helps individuals and organisations sustainably improve their health and performance through practical lifestyle changes. He does this through coaching, public speaking, consulting, podcasting, writing articles, and developing and popularising innovative new products.

Greg’s PhD research focuses on sleep, circadian rhythms, nutrition, and metabolism, and he is co-founder and Chief Science Officer of Resilient Nutrition, a nutrition and supplement company.

Here in England, the highest temperatures ever recorded occurred last summer. And the summer we’re now entering is forecast to be another scorcher. High temperatures challenge our health in many ways, including how well we sleep, which can lead to daytime impairments such as fatigue. Furthermore, many buildings in temperate climates such as England’s just weren’t built to accommodate high temperatures, with their designers more concerned about how to keep inhabitants warm during the winter than cool during the summer. Today, we’re therefore going to consider some practical ways of countering the negative effects of heat on your sleep. Before we get to that though, let’s set the stage by briefly reviewing how environmental temperature affects sleep biology. 

The complex relationship between environmental temperature and sleep

If you reflect on human evolution, it makes sense that very high or very low environmental temperatures worsen sleep, for such extremes could ultimately threaten survival by disposing us to hyperthermia or hypothermia, respectively. Your skin therefore constantly monitors environmental conditions, feeding this information to your brain which in turn modifies how alert you are and your propensity for different stages of sleep. By increasing how alert you feel at extremes of temperature, your brain helps you do things to maintain your core body temperature in a healthy range — for example, seeking shade from the heat or putting on clothes when it snows. 

Your body's temperature undergoes fluctuations throughout the day. Your core temperature, deep within your body, rises and falls by about 1°C over a 24-hour period. It reaches its highest point in the afternoon, around 5 pm, and its lowest point in the middle of the night while you sleep. This fluctuation is controlled by your body's internal clock and helps optimize your body for different activities at different times. When your core temperature is at its peak, you tend to be physically strongest, while it's easiest to fall asleep when your core temperature is at its lowest.

Interestingly, your skin temperature also follows a 24-hour rhythm, but it opposes the pattern of your core temperature. For instance, as you prepare to sleep at night, your core temperature drops while your skin temperature increases. This increase in skin temperature allows for more warm blood flow from your core to your skin, leading to heat loss from your body's core. During colder months, heating your skin before bed can help lower your core temperature and promote sleep. This is why it's advisable to take a hot shower 1 to 2 hours before bed and wear socks while sleeping in colder weather. However, in summer, it can be challenging to lower your core temperature due to the heat in the environment. Therefore, additional strategies are needed to help improve sleep in hot conditions, which we'll discuss next.

Clothes off, sleep on

Starting with bedtime, at the risk of stating the obvious, if you’re struggling to sleep well in the heat and are wearing pyjamas in bed, sleeping naked is probably among the first things you should try. The only part of your body this might not apply to is your feet. When it’s hot, you might want to experiment with sleeping with or without socks on and see which feels most comfortable.

Choosing bedding for the summer

Regarding your bedroom, bedding markedly affects your ability to maintain thermal comfort during sleep. Starting with your duvet, if you overheat under the sheets and use a duvet with a tog rating over 5, try a duvet with a tog rating less than 5. Many of the Fine Bedding Company duvets come in low-tog versions, such as the Breathe® duvet and the Return to Nature duvet. To keep cool, go for cotton sheets, such as the ones in the Smart Temperature range. These use innovative Smart Temp® technology from Swiss textile innovators HeiQ®, making them cool both when you first get in bed and then helping to wick away heat if the temperature later climbs as you lie in bed.  

When it gets very hot, you might benefit from just using a duvet cover with no actual duvet in. Alternatively, you can combine this method with using a duvet over a small part of your body. For example, you can use a low-tog duvet to cover your lower legs up to your hips and then use a duvet cover only over your torso.

Given the roles of brain temperature in sleep regulation, your pillows are key to sleeping in the heat. Pillows differ substantially in how effectively they dissipate heat, so choose pillows that support cooling. There are several pillows available on this site that use innovative technology to optimise the temperature of your head while in bed. Breathe® pillows use a type of breathable fibre named Modal to support heat loss via its moisture-wicking properties and Smart Temperature pillows and pillow protectors use Activated Cooling® technology from HeiQ®. For more on how to find the right pillow for you, here’s our Pillow Buying Guide

Then there’s your mattress. Scientists have found that mattresses that support bodily heat loss increase subjective sleep quality and time spent in deep sleep, so it pays to find a mattress that efficiently offloads heat. While there might be some exceptions to this, spring and hybrid mattresses tend to better aid thermoregulation than foam ones. Foam stores heat, so foam mattresses use cooling pads. But it’s not clear how well these work or how long they last. If you decide to buy a new mattress, get one with a free trial period and a money-back guarantee so that you can give it a fair test run at home. 

If you can’t afford a new mattress or if you just want to preserve your mattress, consider buying a mattress protector or topper that supports temperature management. Using one will help protect your mattress in the long term (for example, from the perspiration you’ll inevitably emit during the hottest months of the year). This mattress protector contains the same HeiQ® cooling technology mentioned earlier, and this Breathe® product is also an excellent option for the summer. Both are washable, making perspiration no problem.

Attend to the air to sleep well in the heat

Air temperature, air quality (humidity, carbon dioxide level, pollution, dust mites, etcetera), and air flow can all influence your sleep. You’ve probably experienced this if you’ve ever slept somewhere hot and humid. People differ in their preferred sleeping conditions, but an air temperature or roughly 18 C is often about right for most of us. In general, a well-ventilated room is ideal for sleep, and some humidity can be helpful, particularly if you have sleep-disordered breathing, for dry air can exacerbate nasal symptoms. 

It’s usually easy to prepare the air for sleep. During the daytime, close the curtains in your bedroom to limit how much the room temperature climbs. Then, unless it’s hotter outdoors than indoors, opening your bedroom window(s) a couple of hours before going to bed will improve ventilation and might be enough to reduce the air temperature to a near-optimal range. As I’ve mentioned in a previous article, there’s some evidence that plants such as peace lilies reduce indoor levels of pollutants, although you might prefer to ignore this if you experience hay fever.

If opening the window isn’t enough, alternative ways of increasing air circulation might help you cool off. Electric fans, for example, increase the evaporation of moisture from your skin, aiding heat loss. Perhaps counterintuitively, scientific research on the use of fans to prevent heat-related health issues is equivocal, and studies on using fans during sleep are a bit mixed too. There is, however, at least some evidence showing that fans can help sleep in hot and humid conditions, so I suggest at least trying either a ceiling or a bedside fan if you’re prone to overheating in bed. If you use a bedside fan, aim it at your torso, not your face. If you have certain allergies, it’s plausible that using a fan could exacerbate allergy symptoms by dispersing allergens, although you might find that a fan that has an air filtration mode counters this problem. If you’re buying a new fan, perhaps seek a relatively energy-efficient one with in-built air filtration. Since allergens and particulate matter can accumulate in your bedroom over time, be sure to vacuum your bedroom and wash your bedding frequently too. 

Use water to keep cool in bed 

There’s been some interesting research showing that, when bedroom temperature is high (32 C), targeted cooling of the back and the neck meaningfully improves sleep quality. And there’s a simple, low-cost way to achieve this. To try it, keep a flannel and a bowl with cool water in it by your side of the bed. Then, if you overheat in bed, dip the flannel in the water and dab your back and neck with it. You can also try draping it over your neck. Over the course of the night, repeat this as necessary.

Keep a glass of water by your bedside too. Your fluid needs rise in the summer when you perspire more, and hydration is important to thermoregulation. So, while some people benefit from restricting fluid intake around sleep (to minimise trips to the bathroom), you might want to keep some water handy in case you wake up thirsty.

Consider sleeping in a different room

This suggestion isn’t an option for many of us, but in case you have the luxury of having several rooms in which you could set up your bedroom, consider whether there’s a cooler room in which to sleep. For example, if you sleep at the top of your home in a room that’s exposed to lots of sun and you have a spare bedroom downstairs that’s exposed to less sun, you might be more comfortable on switching rooms. 

Avoid strenuous exercise in the three hours before bed

While many types of exercise are great for sleep health and evening exercise doesn’t seem to affect sleep much when sleeping in a cool room, it’s probably judicious to avoid strenuous exercise such as hard running, cycling, playing team sports, or lifting heavy weights in the 3 hours before bed. There are several reasons for this, but such exercise is inherently stressful. This stress is good, for it triggers beneficial adaptations to the exercise. However, the stress also acutely increases your body’s stress hormone levels, alertness, metabolic rate, and core temperature, changes which could plausibly be extended by hot conditions. You might therefore benefit from scheduling hard physical activity earlier in the day.

Keep cool, sleep tight!

Sleeping in the heat can be rough, but if you apply the guidance in this article, you’ll be doing what you can to get restorative rest even when it’s so hot that you’re looking forward to the winter. 

This article is meant to complement previous articles on this site about how to improve your sleep, such as the four-part Better Sleep, Better Health series available at these links (1, 2, 3, 4), so do check those out if you haven’t already.

If you have any questions related to this article, you can find us @FineBeddingCo on Instagram, Facebook, and Twitter. We’ll be sharing more summer sleep tips there, so be sure to follow us if you don’t already!

June 01, 2023 — Greg Potter (PhD, MSc, BSc)