When statements like this are made usually it’s because studies would have been done comparing sleep depth at different temperatures and there would have been a statistically significant increase in sleep depth at 18 degrees Celsius in the tested group compared to groups sleeping at other temperatures.

I have no idea if that was actually done here but that’s usually how these statements are justified.

Of course we are all individuals so it may not apply to everyone but you can say that for everything in medicine. Only thing we can do is look for statistically significant differences between groups in double blind controlled studies after controlling for all other factors.

And every time I hear about an optimum, I think of the Bathtub Curve.

As I imagine the curve, I use a real bathtub. Where is the water deepest? At the drain, which is typically just under the faucet. Where is the optimal place to sit? Well, pretty much anywhere except there, because "deepest" (an optimum) isn't enough to accurately gauge "best place". But even if you can swing the faucet out of the way... where is the optimal place? Anywhere on the bottom, a broad, slow curve, as opposed to the steep walls or rim.

So, if 18 degrees C is the optimum, what's the standard deviation? I bet it's not 0.1 degrees. Could be 2 degrees - and that would mean concentrating on 18 degrees is a distraction, not a goal.

This is something evolutionary discussions don't address well, either. If a gene line (species) has a pressure to change, it can "settle" for any place on the bathtub floor (when the solution is shaped like that). This is another example of evolution not seeking an optimum; merely avoiding a pessimum (a word I just googled, and like!).

In fact, it doesn't even matter if it's a bathtub curve. Maybe it's a slope, like the amount of capsaicin a pepper plant carries to fight off fungus. More better - but it costs something to make, so just make enough to ensure survival. More fungus? Increase capsaicin production. There's no floor to this bathtub; it's a linear slant all the way down to maximum possible capsaicin content (which, amazingly, is about 10% by weight of the fruit cells!!!).

18C is more like the maximum OK temperature. But then I also gotta sleep without PJs when its that hot. Still a blanket for comfort.

Best sleep from my experience? It's usually about 5C at the thermostat, which is farthest from the window, by the time I wake up. And that's likely because it's in the basement, which is a typical NA basement, so you have the uninsulated and below the frost line basement wall there. The cold air coming in will be 0C and colder and I sleep next to the window. Winter is my happy sleep time.

>18C is more like the maximum OK temperature.

For you, and that's kind of my point. For me, my preferred temperature would be about 28C with no blanket or a light sheet. Something like 18C can be tolerable with blankets for me, but it's certainly not "maximum comfort", even with blankets. There is too much variation and too many things that wouldn't be controlled for, like number/thickness of coverings, and other such things. Having some "best" universal temperature is just not something generalizable like this.

If you are wearing clothing or any covering, clearly it isn’t your optimal temperature. I’m not trying to be glib here; I understand the appeal of being warm in a cool or even chilly room with air that keeps you from overheating. But that’s not sleeping at the optimal temperature; it’s choosing the sleeping setting you like.

And there are huge swaths of the world where the temperature basically never gets that low (18 C is the lowest temperature ever recorded in Kingston, Jamaica; 16 along the coast of west Africa, 18.9 in Jakarta). Even here in the southeastern US it is normal for temperatures to stay above 20 C at all times for three full months of the year (late June through early September).

Nothing wrong with liking to sleep in cold air, but it’s a preference.

Your feeling might be different. But for me, body temperature and feeling is way different from the air I'm breathing.

I can be, actually want to be toasty warm under the blanket. But I want the air I am breathing to be fresh and cold. That is when I feel comfortable and "at peace".

I will often stick out my feet from under the blanket because it's way too toasty but the rest of my body needs to be warm.

Sure, when it's 25C I'll essentially just lay there without anything but a sheet across the chest to "feel safe" but anything else will just make me way too warm and the air I'm breathing will feel restricting.

What is really great in 25C+ temps is to be outside in a hammock without an underquilt, which cools things down considerably. But I've also slept comfortably at 0F (-18C) in a hammock. With two underquilts and a 0F sleeping bag as well as a winter tarp with doors and snow cover all around, weighing down the tarp sides (wind does suck at that temp ;)).

I was really responding to the article’s odd claim of 18 C being the perfect sleeping temp, and your comment was so extreme in the make-it-cold direction that it seemed to be a good spot to reply. If we’re talking about “optimum” sleep temp as though such a thing has meaning beyond personal preference, then clearly it must not be cluttered with cultural detritus like depth of blankets, etc.

My wife wants it to be 20 C or lower and sleep under two blankets. I pointed out that 23 with a fan and a sheet is less energy-intensive and gives equivalent results, and she said stuff it. Not a battle worth fighting at home, but on the internet…

(Of note, the one of our dogs that loves me but worships her still sleeps next me every night, under covers, because she’s a Chihuahua and gets cold easily, and unlike my wife I sleep like a dead log.)

yeah probably lot of study measurements and data confirming that, I'm not skeptical about it, to me it feels like something scientifically measured proven since quite some time already. of course genetics and outliers still need to be considered.

also do you know that the body temperature decreases during sleep? based on that I simply deduce and assume the more the difference between body and room temperature the more side effects arises for sleep, of course simplifying without ignoring all the rest.

If you want a tried and tested heuristic it should be cold enough to make you retreat to hibernate under a mountain of blankets roughly equivalent to or exceeding the warmth of a polar bear's coat.

Then its good night, Irene