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Enhance evaporative cooling at night
One of the keys to getting a good night’s sleep is to stay cool. But sometimes, due to a lack of air conditioning, or simple biology (eg, menopause), that’s not possible.
For ideal sleep, temperatures from 60 ̶ 67 degrees Fahrenheit are recommended. (15.5 ̶ 19 degrees Celsius.) As we sleep, our core temperature drops. Therefore, this sleep temperature range is far lower than the typical room temperature we prefer during the day.
One technique is to sleep like an Egyptian:
- Sleep under a damp sheet or towel. As the dampness evaporates you keep cool
(Did the Egyptians actually sleep like this or is it another internet myth?)
This takes advantage of a process called evaporative cooling. As moisture evaporates from the surface of your skin (or the sheet on top of you) it takes heat with it. This is also the reason we sweat when hot.
Damp, but not drippy. Throw your top sheet into the shower or use the rinse cycle on your washing machine. To remove the excess water, put it through the spin cycle. Alternatively, use a squirt bottle to mist the sheet first.
For more cooling, you can even throw your damp or misted towel into the refrigerator or freezer. (But don’t leave it in the freezer…)
The best fabrics for evaporative cooling
Your usual 100% cotton sheets will work but they are not ideal. Although breathable, once wet, cotton stays wet. If you’ve just pulled your damp sheet out of the freezer, sure it will feel cool. Temporarily.
But evaporative cooling is lacking with cotton. Dry or wet, cotton is not a good cooling fabric.
Better yet, choose “cooling” sheets made of Lyocell, bamboo (rayon), linen, hemp, or silk. These fabrics may feel similar to cotton (especially after wear and multiple rounds of laundry to soften them up), but unlike cotton, they wick away moisture from your skin.
Once damp, these fabrics dry very fast. (Cooling aside, if your Lyocell bedding is a tad wrinkly, simply use a squirt bottle to mist it with water and smooth it out with your hands. In just a few minutes it will be both dry and wrinkle-free. Try that with your cotton bedding and see how long it stays wet…)
Depending on your cooling needs, using these fabrics dry (as intended) may be sufficient to wick away sweat.
Before replacing your sheets, one option is to purchase a large synthetic cooling towel. Although synthetic, these towels are designed to hold water and facilitate evaporation. They are great for travel or the gym.
Look for towels with an open mesh design that facilitates breathability.
If you tend to exercise or spend time outside in hot weather, smaller cooling towels wrapped around your neck and wrist can provide instant cooling.
Any airflow in the room will improve the evaporation process. An open window, or better yet, a fan. Fans don’t actually drop the room temperature, but they do drop our perception of the temperature by 1-2 degrees. This is because moving air enhances evaporative cooling.
If wetting your entire top sheet is more than needed, try variations, such as a smaller towel on your face and head.
You can also soak your t-shirt or PJs. But again, soaking a cotton tee might not be ideal…
Keep a squirt bottle by your bed in case you need to recharge. (Also good for naughty cats.)
Wet bedding precautions
Before bringing anything “wet” into bed, whether it’s wet hair or a wet towel, make sure your bed is protected from extra wetness. If your bedding stays wet, you risk the growth of mold, which will aggravate allergies.
Assess all your bedding for two properties: cooling and drying. All layers need to do both. If one fails to dry, you can get mold. If one fails to cool, it will hold heat against you, despite all the other cooling layers around you. Fortunately, most fabrics that do one thing also do the other thing.
For example, splurging on new cooling sheets will not help, if your mattress pad and mattress still retain significant heat.
Start from the bottom and protect your mattress. Use a cooling “waterproof” mattress pad or protector. Look for polyurethane (sometimes abbreviated PU).
Older (and cheaper) mattress pads used vinyl which crinkled and famously DID NOT breathe. (We understand if you’ve avoided waterproof mattress protection for this reason.)
Polyurethane breathes and it doesn’t crinkle. In fact, a mattress pad with polyurethane should “feel” no different than one without it.
Purchase a waterproof mattress protector that contains polyurethane and breathable cooling fabric. NOT cotton. Remember, cotton stays wet.
If you have a mattress topper—an extra inch or two of padding on top of the mattress—place your protective mattress pad on top of the topper.
Immediately above the mattress pad place your cooling sheets made of a cooling fabric, such as Lyocell, bamboo, or linen.
Don’t forget about your pillows. What are they stuffed with, and will that stuffing dry?
If in doubt, wet a portion of any questionable bedding and see how fast it dries.
Regardless of the material, if your bed does get wet, make sure it can dry out completely.
It could be your hot mattress
Does your mattress breathe? Traditional mattresses are based on innersprings with a layer or more of foam on top for comfort. The springs are open and therefore hold air, that can aid in heat transfer.
Foam, on the other hand, is designed to mold to your body, preventing heat transfer.
While innerspring mattresses contain only a small amount of foam, on the other extreme, foam mattresses are composed of 100% foam. These foam mattresses are made of layers designed with different properties to provide comfort at the top, and support at the bottom.
Becoming more popular recently, are “hybrid” mattresses with both springs and a significant amount of foam.
As expected, the 100% foam mattresses tend to be the hottest.
You may have heard of memory foam, which is made from polyurethane treated to increase viscosity and elasticity (“viscoelastic foam”).
The advantage is that it conforms to your body and is great for pressure relief. It’s also great for couples as there is little to no transfer of movement. But memory foam is notorious for holding in body heat.
Even if you don’t sleep on a “memory foam” mattress there may be a layer of this stuff within your mattress sandwiched between other foam types.
Fortunately, there are some advancements that are helping. Newer mattresses are incorporating these products into some of their layers
- Gel memory foam, which contains tiny beads of gel
- Foam with other cooling minerals, such as graphite or copper
- Foam with larger air pockets. Any space that can hold air will improve circulation and therefore heat transfer.
Yes, these improvements definitely help with the problem of hot memory foam, but my personal experience is that more or less any option that doesn’t include memory foam is still cooler.
Although traditional innerspring mattresses and hybrids are reported to be cooler, there is significant variation among all mattresses, based on the composition of the foam layers closest to your body, in combination with the airflow from the support layers.
Many mattresses will market whether they are “cooling” or not. Unfortunately, side-by-side comparisons are limited. Ultimately, it will come down to individual preference.
It could also be your bed
In simple terms, airflow aids cooling. Preferably your bed should be off the floor and have wooden slats, rather than solid wood, below the mattress.
If you still use a box spring mattress, they are composed mostly of air and should work fine.
But to be honest, if you sleep on a thick foam heat-absorbing mattress, airflow a dozen inches below will probably have little impact.
Ditch your bed completely
If your heat issue is temporary, perhaps during a heatwave, then consider temporarily relocating your sleep location.
Heat rises, so if you sleep on an upper floor move downstairs. Assuming your finished basement has egress (a way to escape in case of fire), set up a temporary bed there.
Another option is an air mattress. Newer models are as thick as conventional mattresses and come in all sizes including queen and king. Most come with an air pump installed. Simply plug them in and flip the switch. They will fill in less than thirty seconds.
The added advantage is that you are sleeping on air which has the best heat transfer. (You may even be able to stay in your bedroom, assuming you have floor space.)
Another option is to set up a hammock to sleep in. Set-up indoors may be a bit of a challenge but maybe a fun option for a kid’s room. Again, this option provides plenty of airflow underneath where you sleep.
Other tricks to sleep cool in hot weather
- Shower and wet your hair prior to turning in
- Take a hot shower or bath before bed. Yes, this sounds counterintuitive, but the hot bath will help lower your core body temperature and help you fall asleep faster.
- Flip your pillow over. Or keep an unused one nearby.
- If you have long hair, pull it into a loose ponytail held either at your neck or the top of your head
- Create an icepack from a hot-water bottle or simply two zip-lock bags wrapped in a towel
- Run your wrists or feet under cold water briefly before going to bed
- Keep your bedroom as cool as possible by closing room-darkening drapes during the day
- If the outside cools down at night, consider installing a whole-house fan. These fans live in your attic, pull cool night air through a cracked window, and push it out through the hot attic.
- If you aren’t ready for the whole-house fan investment, place a window fan “backward” in a window on the opposite end of the house, while cracking your bedroom window. This works on the same principle: the fan will push hot house air out, creating a vacuum that pulls cool night air in through your cracked bedroom window.
- No central AC? Perhaps you can temporarily install a window AC unit. Yes, there are also portable AC units, but avoid anything that requires you to add water. Those are not “air conditioners”, but instead are swamp coolers and they only work in very dry climates. If a window AC won’t work, look for portable units with not one, but two hoses (one inflow, one outflow) that can be inserted into your window.
- If money isn’t an issue (but for some reason you can’t have AC), consider a water-cooled mattress pad. These units send cooled water at the exact temperature you specify through tiny tubes below you.
- Always keep a glass or bottle of water near your bed. Stay hydrated.
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