Why are sheets colder in winter when the room isn't?

We generally keep our house at least as warm in the winter as we do in the summer. That said, the sheets are darn cold on the coldest days of the year. It’s as if the sheets know what the temperature is outside, despite the toasty warm indoor temperature.

I accept this could be my imagination as I have no data to prove my theory. I also accept that my sheets are (probably) not sensient. But I’ve raised the question with others and they concur with my observations.

So why do some of us perceive that sheets are colder on the coldest nights, even when the room temperature isn’t perceptively different?

Specific heat, also known as heat capacity. Since air is less dense than the sheets, it cannot store as much heat energy. Although the temperature of both the sheets and the ambient air are the same, the sheets feel cooler because they can “suck” your body heat away faster. Similarly, water at 4C will cause hypothermia well before air at 4C will.

Sorry to double post, but density is directly related to specific heat, and also to the heat transfer rate. Analogously, sound travels faster in more dense materials, as does heat. Since the sheets are more dense, they can both move the heat faster, and store more of it. Since more of your body heat is gone, you feel much cooler.

I think you are wrong about what temperature you keep your house in the winter, unless you use a hell of a lot of air conditioning in the summer, or a hell of a lot of heat in the winter.

Most people tend to dress more heavily in the winter, and thus keep their house colder. Since you’re dressed in thick layers, you feel normal - but when you take off your clothes and go to bed, you notice.

No way. It happens even if you’ve been hanging out in your boxers for awhile on top of the covers.

I would suggest that it is completely psychological. Certainly in temperate Britain in a house with central heating, the indoor temperature does not vary much season to season. It is rarely hot enough outside to raise the temperature in the house during the summer and most people seem to heat their houses to about 19C- 66F or so. So temperature is similar all year.

Psychological explanation:

Summer- hot outside, aware how nice it is to cool down a bit. Get into bed- what lovely **cool **sheets!

Winter- cold outside, aware how nice it is to be warm. Get into bed- what horrible **cold **sheets!

Where **cool **equals cold objectively, but subjectively they feel different.

While those observations are generally true, I very strongly doubt that’s what is going on in this instance. I’ve used actual thermometers to see the OPs phenomenon. It’s not subjective. I think Pygmy Rugger’s got the right of it.

Have you actually measured the temperature of the sheets during the summer and during the winter and compared each of those measurements with the ambient temperature?

If not, then I don’t think that your data is useful.

IMHO if you have a bedroom that is say, 65F in winter and in summer and this is the case all day, then the sheets will be the same temperature, winter or summer.

However if the temperature varies (un air conditioned house in hot climate in summer) and the temperature in the bedroom rises to say 85F during the day and falls back to say 65F at night, of course the sheets will be warmer.

So either the statement in the original post “We generally keep our house at least as warm in the winter as we do in the summer” is partially wrong, or the effect is entirely psychological.

If your room temperature fluctuates between, say, 16 C and 25 C, small objects left exposed will tend to equalise with the current ambient temperature pretty fast. Insulated objects, such as the inside of your bed, and larger objects, such as human corpses, will take time to approach equilibrium - and will therefore tend to always be warmer than the coldest end of the temperature range, but cooler than the top end. They will tend to remain at the average room temperature.

But that still doesn’t address the OP about sheets seeming to be colder in winter whilst he/she insists the temperature is the same in the house in winter and summer. If the OP is wrong about his/her house temperature then your explanation is valid. If he/she is right, then psychology is the only possible explanation.

I suspect it’s a little of both.

Just to provide an additional example to PR’s explanation, which I think gets it right, touch a marble surface and a piece of wood both at the same temp. Marble feels colder than wood for very same reasons It just sucks your body temp better.

What confuses me is that you say that it happens even afer you have been hanging out on top of the bed cover. Unless we are talking about a very good cover or a very short time, you should have brought the sheets up to temp already.

I suppose it 's the same as the old cold-side-of-the-pillow thing - human heads are quite warm (at least as long as they remain attached to a living human body) - yet the underside of a pillow is still perceived as quite cold even after a full night’s sleep on it.

It’s radiation. Your bed ‘top’ ‘sees’ the ceiling, which has either the attic or a unheated air space above it. The temperature of the ceiling radiates down to the bed. The air temperature near the ceiling doesn’t radiate, just solid objects.

This is the same reason why on a cloudless night it gets very cold, the ‘ceiling’ in that case is space, which is very very cold.

I think that means in bed, no one can hear you scream.

You are correct in assessing my indulgence – we use a hell of a lot of heat in the winter and hell of a lot of air conditiooning in the summer. With some exceptions, we keep the temperature in the house around 69-71 F year-round. (The house is tightly built by any standard I’m aware of, and I’m a big fan of central heat and air.)

For clarification, we also dress the same for bed in summer as we do in the winter. But we do tend to wear our warmest clothes on the coldest days, so the difference in attire when getting ready for bed is greatest on those most-frigid nights.

I dunno, but if you get cotton flannel sheets, the temperature differential (actual or merely perceived) isn’t nearly as noticeable, and you’ll sleep more comfortably in the winter, too.

No. There’s no such thing as cold, only lack of heat. And lack of heat can’t radiate.
I agree with the flip-side-of-the-pillow bit, though. It doesn’t take much to insulate the 30F difference between your body and the sheets, which will always be at basically ambient temperature.

Ditto on Sapo’s explination about the wood and marble at the same temp.

I think Mangetout has it, pretty much.

Unless you have your heating going all day then the temperature of the room in winter is going to drop lower than average for at least part of the day. E.g. I have the heating coming on a couple of hours before I get up and a couple of hours before I get home. While I’m asleep, or at work, the house is going to cool down. When the heating comes on, the air quickly warms up to the normal temperature, but the sheets, by virtue of their greater heat capacity, will stay cold to the touch.

And Pygmy Rugger, with the heat capacity part.