Wednesday, May 28, 2014

The Science of Sleeping Warm (without raising your electricity bill)

The year we moved to Melbourne, I completely froze. No matter how many pairs of socks, no matter how many blankets, I just couldn't get warm when I went to bed. It was miserable. I've tried all kinds of strategies in the six years we've lived here but haven't really hit on any winners that didn't involve an electric blanket. Even using an electric blanket for just 10-15 minutes to take the edge off the chill can really make the numbers leap ever higher on the electricity bill. When Dear Boy came along and I was guilted by all the mama-literature into keeping his room at a constant 18 degrees, all winter long (and boy is it a long winter) - well the electricity company was happy, but I was less so with the bill.

Well, this winter, I'm aiming to keep the bills low and my toes toasty warm. And in that spirit have set out to investigate the science of sleeping warm. As always, this is part science, part in-depth research on the internets and part realisation of things that probably should have been common sense. I learned more about blankets and layering that I thought I ever would.

Here's our heating/temperature baseline so you understand where I'm coming from and adjust any advice you glean to your own circumstance:

  • we live in a 1940s brick house with nothing fancy added (think fridge in winter and oven in summer)
  • we have no central heating
  • we have gas heaters in the living areas - one of which is fantastic while the other is largely ornamental.
  • we've used an oil heater in Dear Boy's room because they're more kid safe than other types and don't blow dust around; they are, however, terribly costly to run. 
  • we have very high ceilings
  • we have single pane glass in the windows, supposedly 'thermal' blinds and then thick curtains.
  • we have gaps under the doors that are too high for conventional 'snakes'.

So it's cold okay.

I'm going to break down the ideas I found for keeping warm into two categories: the body (and things you can do before getting into bed) and then the bed and it's environment.

The Body

Okay - science first. Did you know your body loses heat in four different ways? These are conduction, convection, radiation and evaporative cooling.

Conduction means you transfer heat when touching something else. For example, if you lay on the ground, your warmer body transfers its heat to the colder floor. The same thing happens if your sheets are colder than your body. Convection means air circulates, moving heated away and replacing it with cooler air. For example, a breeze blowing over you steals a tiny little bit of your warmth and gives you cool air in return. Radiation is a bit freaky, but apparently our bodies are constantly emitting electromagnetic waves mostly from the head (yeah, that's the freaky part). This is/causes heat loss. And evaporative cooling - yep, that's your awesomely efficient body at work, releasing sweat which then evaporates, taking the heat with it. It's not just sweat though, anything wet has the same effect.

So, the idea is to prevent or minimise this happening and to boost the body's methods of producing and distributing heat. Your metabolism, for instance, is still hard at work, even after you stop eating and head to bed, chugging away to burn those calories. If you don't have enough calories on board, it's harder to produce heat - so extreme diets are out if you want to stay warm at night. Things like spicy foods and ginger can speed up the metabolism and help keep you toasty. So can a diet high in fats - but that's assuming that your metabolism is actually going to burn it, not just pad it straight onto your butt - so I'd avoid using a fatty diet as a method for staying warm unless you already have a high revving system on your side. Take it from me, a few *cough, cough* extra kilos do not help you sleep warm. In my experience, they can actually keep you up at night.

Another body trick to know is that when your core is warm, it sends more warmth to the extremities. If you suffer from cold hands or feet at night, think about adding another layer to your core (a singlet, for instance). I've tried this a few times now (very scientifically, of course with very many controls, and a full ethical clearance for human research - okay, not really) and it's now my go-to solution if my feet are cold - add an extra layer up top instead of another pair of socks. More than two pairs of socks just tends to cut off the circulation anyway (which of course, makes your feet feel colder, sigh).

Other pajama advice is, if layering, to wear animal fibres or cotton in the layer next to your skin. Pajamas/clothes made from these fabrics are more likely to keep in the heat and absorb moisture, moving it away from your skin so it doesn't evaporate on your skin and cool you down.

Still in the 'what to wear to bed' category, the jury is out on whether or not a hat (beanie) of some kind is a good idea. Yes, it'll keep you warm (keeping in all that crazy radiation after all), but the question is whether you should trap quite that much heat and run the risk of over-heating - bit of a delicate balance, really. From what I've read on the general science and camping/extreme hiking sites, hats should be reserved for bald folks or more extreme weather conditions (i.e. camping in snow). Those of us with a good head of hair are trapping some heat anyway.

Once PJ-glad, my general strategy for getting warm in the past has been to blast my electric blanket on its highest setting for 10 minutes, slide on into that toasty warm bed and defrosting enough to lose consciousness. Because I'm trying to keep that (and my electricity bill) to a minimum, I'm going to pull out my trusty old hot water bottles. I have never, ever been a fan of these after a friend had one burst in the night and scald her legs. I certainly wouldn't leave one with Dear Boy but I think they do a place for a good pre-warm session, taking the chill off the bed so you don't lose all your heat via conduction into the stony cold sheets and mattress.

Giving your body a quick jolt of heat before bed can also help keep you feeling warm once you're all tucked in. A quick shower, a few minutes basking in front of the heater, or drinking a hot herbal tea can all raise your core temperature by enough to warm your extremities and help you drift off to sleep. If you're showering before bed, though, do make sure you dry off really well and give yourself a blast with a hairdryer. Sleeping with wet hair or even slightly damp PJs can rev up your personal evaporative cooling system. Save that one for summer.

Lastly, make sure you pee before bed. Once you're in bed and toasty warm, there's no surer way to lose all that body/bed heat than having to emerge from your cocoon and use the bathroom (I swear our toilet seat is the coldest in the world).

Quick take-away tips for the body:

  • eat spicy foods/ginger generally to get the metabolism firing
  • add layers to your core for heat the extremities
  • wear animal fibres or cotton close to the skin
  • wear a beanie if you're bald or super cold
  • pre-warm the bed with hot water bottles or hand warmers
  • warm up the body with a quick shower or a hot herbal tea
  • dry off before bed
  • pee before bed

The bed and bedroom

Now for the environmental factors at play. Bed type and placement can be making your colder than your think. External walls are generally thicker and colder to the touch than internal ones so, where possible, it's better to line your bed against an internal wall so you're less likely to conduct heat to the colder wall. Having a bed directly on the floor can have the same effect. Better to have a bed raised off the ground (on a bedframe or ensemble box) so you're further away from the cold floor. Even when you're raised up, mattresses can still steal quite a lot of heat away from your body - adding a blanket or woolen underlay under the sheet can help to prevent some of that.

Now, to the top of the bed. Blankets. Layers. We know these things are good but how the hell do they work best?

The science? Air is a great insulator when it's relatively stationary. Even though it can cool you down by moving warmth and replacing it with cool (convection) and helps to evaporate sweat (evaporative cooling), when it's trapped between layers and not moving over the skin, it helps prevent heat loss.

Some bedding take advantage of this kind of insulation quite well already. Quilts, like the kind I made (excuse that shameless opportunity to relieve a sense of achievement), layer thin fabrics with batting and therefore create warm air pockets within the quilt. The same goes with feather or down doonas (duvets) where the spaces between the feathers or down create air pockets (apparently goose down is the warmest of all). The pockets of air in these blankets help to trap heat more effectively than thinner, denser blankets - essentially they don't conduct heat away as well.

But! There's a but! Denser blankets help to slow convection to the outside. I am assuming this is because of the density of the blanket make it more difficult for the air moving past it to take heat and replace it with cooler air. I'm still looking into that one, but the crux of it is, layering a denser blanket on the outside of your layers is better than having it closer to the skin. If you are using multiple layers, keep the fluffy ones like quilts or doonas closer to your body and then place denser blankets on top (like a shell).

But! There's another but! I know it seems completely counter-intuitive, but don't add too many layers or you'll squish the air pockets in your fluffy layers. By crushing all those lovely air pockets, you're losing all the benefits of using air as an insulator or killing the effectiveness of your lower layers at preventing heat loss.

One last trick - and it's still on the topic of air - is to minimise the amount of space around your body where air can move freely around, passing over your clothes and skin and cooling you down and just generally minimising the amount of air that needs heating. I'm a big fan of cocooning - or the burrito method - where I shuffe around so the blankets are lightly trapped on each side of my body or tuck the blankets in around my boy. It minimises those pesky gaps where cold air gets in. Another method (a trick from campers) is to fill the gaps with your spare clothes, again reducing the amount of air around you that also needs to be heated.

Quick take-away tips for the bed and bedroom

  • Line your bed up against an interior wall and raise it off the floor
  • Use a blanket or woolen underlay over the mattress
  • Layers are tops
  • Quilts and feather/down doonas have air pockets that are great insulators
  • Put fluffy/airy layers underneath and thinner, denser ones on top
  • Don't have too many layers or you lose the benefits of air as an insulator
  • Tuck yourself in or stuff the gaps around your body with spare clothes

Are you a cold-frog like me? How do you keep warm in winter? If you live somewhere particularly frigid, please share all of your worldly experience. 


  1. I loved this post. I'm taking notes. We live in an old weatherboard with high ceilings and we FROZE the first year we moved in. The single best purchase we made in that first year? Dressing gowns for everyone. We hadn't needed them in our last house!

    1. Ooh a good dressing down is the best. On some of our coldest nights, I'd wear mine to bed or have it draped over my feet. The things we do to stay warm!

  2. Carpets and rugs on the floors, layering insulation in the room. On the bed, try a quilt underneath you, especially a down one as it will "loft" around you a bit creating that cocoon effect. And make sure your chest, lower back and back and shoulders are warm. Whilst breast feeding I slept with a fleece jacket over and with a merino singlet under my pjs.

    1. Ooh the doona or quilt underneath sounds like a good idea. I like the sound of 'loft'.


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