Sleeping Cold


Jacob Schor, ND, FABNO

Far back among memories from my youth, there is an earnest conversation I had with a young woman when we were 16.  She asked me where I saw myself in the future.  In a flash of prescience, I described a small cabin surrounded by forest and sleeping in a warm and snug loft above the fireplace.  In retrospect, that was the wrong answer.  The response she sought was probably more about colleges, majors. That wasn’t the first or last time that I’ve said the wrong thing to the wrong person at the wrong time.  My description of the cabin on the other hand may have been a brief flash of foresight as, now, half a century later I find myself living in the cabin I had described. 

I realize now that I was wrong about one thing. Sleeping in the ‘warm and cozy loft’ does not provide a good night’s sleep. Hot air rises and it’s too warm in the loft.  Only on the coldest of winter nights is the warmth appreciated.  But on those nights, someone has to climb down the stairs to put wood in the stove during the night.  My vision of a cozy loft was off.   Much has been learned in recent years about what it takes to get a good night’s sleep.

Perhaps the most important thing learned is about sleeping temperature.  We sleep better when cold.  It’s a Goldilocks thing, “Excessively high or low ambient temperature may affect sleep even in healthy humans without insomnia.”1 We don’t want it too hot or too cold, but what is just right? This has been a challenge to study.  Laboratory animals sleep nude, often without bedding covering them; scientists have no murine pajamas. Circadian rhythms, sleep onset, and thermoregulation are closely tangled together so that what effects one shifts all three.

One’s core body temperature drops during night-time sleep.  It is hard to fall asleep if your core temperature is increasing.2 Decreasing body temperature makes it easier to fall asleep, and deep sleep allows one’s temperature to drop even further.  Sleep allows relaxation of the noradrenergic vasoconstriction in the skin, allowing further heat loss as increased blood flow transfers heat from core to skin.3 Skin temperature stays high even as one’s core temperature drops. In fact, if you trigger vasodilation in “distal skin regions” (think hands and feet), you hasten melatonin secretion and sleep onset. In practical terms, wearing warm socks in a cold bed might help you fall asleep faster.4 I suppose that might apply to mittens as well but haven’t located a study that tried it.  Interestingly, if the temperature of the skin on one’s chest is allowed to drop, sleep falters. This is especially true in older people.5

We want our core temperatures to decrease at night.6 The body reacts to even small increases in sleeping temperature as if it were a stressor.  Sleeping warm raises morning cortisol and reduces sleep quality.  In the elderly this leads to an overactive sympathetic nervous system.7 Elevating cortisol might be experienced as waking alert and anxious in the middle of the night.  This isn’t a valerian deficiency or an indication for homeopathic Arsenicum, or anything else we may have tried over the years with such patients.  It’s simply overheating from sleeping in too warm a room or with too warm a blanket.

There’s something of a microclimate created under your covers at night.  Both temperature and humidity act together to create a comfortable bed to sleep in.8 Apparently the ideal conditions are 90 to 93 degrees F (32-34° C) and 40 to 60% relative humidity.9

This is complicated by the fact that while most of our body is snug under the covers, most of us keep our faces exposed.  Perhaps this a leftover habit of night-time watchfulness against threat, preserved because it provided a survival advantage?  Cold stimulation on the face triggers a specific cardiac parasympathetic activity and induces bradycardia.  Sleeping in the cold causes cardiac parasympathetic activity to dominate the sleep cycle.10

We want to stay snug at night with our faces uncovered and without so many covers that we wake up sweaty.

Heart attacks peak during the winter months and some of these changes in heart function during sleep may explain this.  Ischemic disease mortality isn’t associated with outdoor temperatures but rather with living-room temperatures and poor bedroom heating in the winter.11 Cold climates increase blood pressure and blood factors that increase risk of thrombosis.12 Heart rate variability shifts dramatically between sleep stages, and between sleep and wakefulness, more so if one is too cold. In fact, the current scientific advice is that we should avoid sleeping in rooms with an ambient temperature less than 50° F (10° C). It’s that Goldilocks again, not too hot, not too cold.

During the day, we can regulate body temperature by shifting locations, adjusting clothing, or adjusting the thermostat. This is all harder while asleep. Adjusting the covers takes effort.  Interestingly, the upper extremities have a greater sensitivity to changing heat than the lower extremities in how they regulate sleep.13 Sleep position also changes heat. Lateral position (that is sleeping on one’s side) reduces heat exposure compared to laying supine.14

If too much heat leads to poor sleep, being hot and humid at the same time is even worse. Humidity increases the effect heat has on sleep, increasing wakefulness, decreasing rapid eye movement (REM) and short-wave sleep (SWS).  Excessive humidity prevents the body’s core temperature from dropping at night.  Hot and sweaty is not the goal. If sweat doesn’t evaporate, the body core temperature doesn’t drop and one can’t sleep well. Sensitivity to this humidity is greatest during the initial segments of sleep and drops off later. One stays up late in the summer waiting not just for it to cool off but probably knowing that sleep will be easier later when sensitivity to heat and humidity decreases.  

Body temperature is considered to be one of the zeitgebers that set the circadian clock.  The main influence on the clock remains light exposure, which signals the suprachiasmatic nucleus in the hypothalamus; but the list of lesser influences has grown to include temperature, habitual eating times, and exercise times.

There is something else about our understanding of what contributes to a good night’s sleep that has changed.  Down quilts that once were so sought after are now out of style.  Not only are they too warm for a good night’s sleep, but down quilts aren’t heavy enough to promote sleep.  People want weighted blankets now.  Initially weighted blankets were suggested to help kids with disabilities sleep better.15 Then improvement was reported for patients with dementia16 and psychiatric disorders.17 At this point everyone is trying them.  The research suggests one choose a blanket that weights about 10% of your body weight although a 2006 study reported 30-pound blankets were particularly effective.18 To me that seems like too much of a good thing.  For comparison, the lead apron draped over you at the dentist during x-rays weighs about seven pounds.19

This heavy blanket idea isn’t limited to helping deeply disturbed people with diagnosed psychiatric pathologies; a 2020 study reported weighted blankets were effective at reducing anxiety in patients undergoing cancer chemotherapy.20 However, before we consider this therapy to be well proven, we should note that a 2020 meta-analysis that combined data from eight studies concluded the evidence shows “… that weighted blankets have the potential to be beneficial in limited settings and populations.” The blankets may “… be an appropriate therapeutic tool in reducing anxiety; however, there is not enough evidence to suggest they are helpful with insomnia.”21 This conclusion contrasts the oft cited 2015 paper by Ackerley et al that suggests these blankets help treat insomnia.22 This later paper does not appear to be listed on PubMed.  Apparently, the relatively new journal, which published it, only started being indexed in 2016.  Thus, these blankets may fit into the category of “Try it, you might like it”—for the time being.

Next time I dream up a cabin, I’m going to put more insulation in the roof and walls, use the loft for storage, and add both a summer sleeping porch and a bedroom downstairs.

References

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About the Author

Jacob Schor, ND, now retired, had a general practice with a focus on naturopathic oncology in Denver, Colorado. He served as Abstract & Commentary Editor for the Natural Medicine Journal for several years (https://www.naturalmedicinejournal.com/) and posts blog articles on natural therapies,  nutrition, and cancer (https://drjacobschor.wordpress.com/). He is a Board member of CoAND and past president of OncANP, and someone who is happier outdoors than inside.