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I can completely relate to your question @irelandscape. I have noticed this too. And not just about warmth during sleep, but also she prefers warmer room than I do. She prefers warmer water during shower than I do. Your questions made me do some reading and ask some questions. I think I do understand it slightly better now. A complete understanding of biology which psychology of thermal comfort, still doesn't exist, however we do have some vague idea. But before even try to look at the biology, we need to ask if we both are just observing anomalies or is something really going on? So let's start there.

Me: So how does a warm cozy evening by the fireplace, with hot cup of coffee and a good book to read, sounds like?
She: I suppose, perfect
Image by jill111 | pixabay license.

Gender differences in seeking warmth

In fact it is seen that women do prefer about 2.5 degrees warmer temperatures than men (Kingma et al., 2015, Lian et al, 2008). Also, men and women seem to differ in their temperature tolerance. For instance Volceker, 2015 found, that while both men and women sense cold alike, women have less tolerance for cold. Same is true for heat, as well (Averbeck et al., 2017). This indicates that women are more sensitive to temperature and might have different thermal comfort zone.

However, what I found even more interesting is that, this dimorphism in temperature preferences and difference in tolerance levels, are not limited to humans. For instance, female mice prefer to reside at temperatures around 32°C, while male are not that picky (Kaikaew et al., 2017). In both, rats and humans females are more sensitive to extreme heat and cold. But given the fact that females show higher vasoconstriction following a painful stimuli, researchers predicted that females will not find prolonged heat exposure as aversive as they would find prolonged cold exposure. Well, torturing humans as much wont be considered ethical, but Vierck et al., in 2008, did test it, in rats. When they placed the rats in a situation when half the floor was heated and half was cold, they found females found cold more aversive than heat. And males found heat more aversive than cold.

But why?

Women are cold, I mean their hands are cold

Alright then, we have established that temperature preferences can be sex dependent. However, why this would be the case still eludes us. There two interesting explanations out there. But crux of both rely on skin temperature differences between males and female. Vierck et al., in 2008 noted that while core body temperature in rats did not differ between males and females, females do have lower peripheral temperatures. Similarly, in humans when men and women are exposed to cold, women have 3°C lower skin temperature as compared to men (Kim et al., 1998).

A matter of Hormones

The lower temperature can in part be explained by the fact that estrogen thickens the blood. This reduces the amount of blood flow in the capillaries, at extremities. Hence, when exposed cold women may find it harder than men to warm up the hands. I say in part because this when Kaikaew et al., 2017, removed the gonads of mice it did not change the female preference for warmer temperature. Hence, while this may explain why women may not like cold as much, it fails to shed light on why women would pick warmer side of temperature in normal range. Probably there is something more permanent. Something pre-programmed in body and brain, during development or even years of growing up.

A matter of metabolism

Metabolic process inside body constantly produce heat. So, if you are in surroundings at a slightly colder temperature than your body, the thermostat in your body (aka the hypothalamus) would ask your cells to generate more heat. But, what if you have lower metabolic rate? I think, in that scenario one good strategy would be to stay at slightly higher temperature. This would give your body a head start if outside temperature starts dropping. Well, an extreme case would have been if you were cold blooded organism, like a crocodile. They solely depend on external temperature for their body hear regulation. However, humans are not cold blooded. They can however vary here and there in their resting metabolic rate. Based on this we may predict that slight help from environment may come handy.

But do female have lower metabolic rate than males? Well in absolute terms it appears that men have higher resting metabolic rate than women. A majority of this difference can be explained by body fat composition (Buchholz et al., 2001).

Let's make a prediction. Let's predict that if we pick other groups differing in metabolic rates, then we may find differences thermal sensations and even comfort levels. One such group will be of old people. As we age, our resting metabolic rate decreases. And, studies have shown that elderly have decreased warmth sensitivity in cold season and decreased cold sensitivity in summers (Tsuzuki et al., 2002)

However apart from metabolism, this can also be explained by (1) impaired ability to regulate skin temperature, (2) impaired vasoconstriction and vasodilation
(3) slower nerve conduction and decreased peripheral nerve fibres in old age (Guergova and Dufour, 2011).

This make me wonder that we totally forgot to look at neural correlates of thermal preferences in males vs females. Hmm!

The thermal comfort zone in the brain

Where in the brain is thermal comfort zone at?
Image by John Graner | Public Domain

If we are sensing the temperature of our surroundings and regulating our body temperature in response, there has to be neral correlates for this mechanism. We already know the regulator thermostat - the hypothalamus. But it would be more interesting to find the neural network that converts the physiological sensation to psychological feeling of comfort. And, if we know that we can question how this network differ in males vs females.

In this regard, Hajime et al., 2017, did make an attempt. They played around with ambient temperature, while their subjects had their brains being scanned in fMRI. They found following areas responded to temperature sensation - dorsal posterior insula, putamen, amygdala, and bilateral retrosplenial cortices. However, due to technical issues, they were not able to elucidate the correlates of thermal comfort.

But nevertheless, their findings were still interesting. For instance Dorsal posterior insula is also the primary thermosensory cortex, found in monkeys. It is in fact novel in evolutionary timeline. But, yet more interesting is involvement of amygdala. This is because (1) amygdala has been previously tied to emotional response to thermal discomfort , when exposed to cold (Kanosue et al., 2002); and (2) amygdala is sexually dimorphic (Cooke et al., 2005). Now weather, this dimorphism can explain differences in thermal comfort feel, I don't know. But it is a question worth asking.

In a nutshell a lot remain unexplored in this arena. However we do have hints of what might be going on. Once we understand what, we can do mental masturbation about why. Until then lets keep talking and promoting good science on @stemq and @steemstem.

Cheers!

References

1. Kingma et al., 2015. Energy consumption in buildings and female thermal demand

2. Lian et al., 2008. Investigation of gender difference in thermal comfort for Chinese people

3. Voelcker T. Perception of cold hands: comparison between women and men. Extrem Physiol Med. 2015;4(Suppl 1):A80. Published 2015 Sep 14. doi:10.1186/2046-7648-4-S1-A80.

4. Averbeck B, Seitz L, Kolb FP, Kutz DF. Sex differences in thermal detection and thermal pain threshold and the thermal grill illusion: a psychophysical study in young volunteers. Biol Sex Differ. 2017;8(1):29. Published 2017 Sep 1. doi:10.1186/s13293-017-0147-5

5. Kaikaew K, Steenbergen J, Themmen APN, Visser JA, Grefhorst A. Sex difference in thermal preference of adult mice does not depend on presence of the gonads. Biol Sex Differ. 2017;8(1):24. Published 2017 Jul 11. doi:10.1186/s13293-017-0145-7

6. Vierck CJ, Acosta-Rua AJ, Rossi HL, Neubert JK. Sex differences in thermal pain sensitivity and sympathetic reactivity for two strains of rat. J Pain. 2008;9(8):739-49.

7. Kim et al., 1998. Cold hands, warm hearts.

8. Buchholz AC, Rafii M, Pencharz PB. Is resting metabolic rate different between men and women? Br J Nutr. 2001 Dec;86(6):641-6. PubMed PMID: 11749674.

9. Tsuzuki et al., 2002. THERMAL SENSATION AND THERMOREGULATION IN ELDERLY COMPARED TO YOUNG PEOPLE IN JAPANESE WINTER SEASON

10. Guergova and Dufour, 2011. Thermal sensitivity in the elderly: A review

11. Oi H, Hashimoto T, Nozawa T, et al. Neural correlates of ambient thermal sensation: An fMRI study. Sci Rep. 2017;7(1):11279. Published 2017 Sep 12. doi:10.1038/s41598-017-11802-z

12. Kanosue K, Sadato N, Okada T, Yoda T, Nakai S, Yoshida K, Hosono T, Nagashima K, Yagishita T, Inoue O, Kobayashi K, Yonekura Y. Brain activation during whole body cooling in humans studied with functional magnetic resonance imaging. Neurosci Lett. 2002 Aug 30;329(2):157-60. PubMed PMID: 12165401.

13. Cooke BM, Woolley CS. Sexually dimorphic synaptic organization of the medial amygdala. J Neurosci. 2005 Nov 16;25(46):10759-67. PubMed PMID: 16291949.

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