How to Fall Asleep Faster: Temperature and Sleep

Temperature and Arousal: Evolutionary Significance

I’m sure you’ve noticed how hard it is to sleep when you’re too hot or too cold. When you think about it, this makes evolutionary sense. Reproduction is the most primordial human drive. Survival is necessary to reproduction, and to survive, you must adapt to your environment.

As the core of your body is deep within you, it is somewhat thermally buffered from the outside world. In other words, even if environmental temperature changes a lot, your core temperature may barely budge. However, your body can only tolerate a certain range of external temperatures before your core temperature shifts substantially and serious consequences ensue.

Imagine yourself in the Arctic tundra. Naked. If you don’t find insulation, your core temperature will plummet, hypothermia will rapidly set in, and you will pass away. It therefore makes sense that your skin constantly monitors environmental temperature to inform your brain about external conditions. In response, your brain modifies your arousal so that you can thermoregulate (by changing your clothing or shelter, for example). If it gets too hot or too cold, you’d better not be sleeping!

Now, humans are endotherms. This means that we produce our own heat, allowing us to maintain relatively stable body temperatures, even when we’re exposed to highly variable environmental temperatures. Endotherms evolved from ectotherms – organisms that rely mostly on environmental heat sources to regulate body temperature. (When you see reptiles basking in the sun, they aren’t trying to tan.) Ectotherms need heating to be active, and they sleep when their whole bodies are cool. But we endotherms are a complicated bunch. Like ectotherms, our core temperatures fall during sleep. But our skin temperatures actually tend to go up at this time.

We instinctively do things before sleep increase skin temperature, the most obvious of which is creating warm microclimates (I love my duvet). Interestingly, some of our behaviors specifically increase the skin temperature of our extremities, and skin warming in these regions is particularly conducive to quickly falling asleep. These skin-warming rituals include turning off the lights as well as lying down – that’s right, one way lying down permits sleep is by increasing the temperature of your hands and feet.

But how is this all regulated?

How Temperature Affects Sleep

Every 24 hours or so, your core temperature predictably varies a little, and it’s easiest to doze off when core temperature is near its trough (1). Now, the temperature at the surface of your skin determines how much blood flows through blood vessels close to your skin, and higher skin blood flow leads to more heat loss from your body’s core.

The volume of blood passing through the vasculature of your hands and feet is particularly relevant to sleep. The reason is that these body parts are remarkably dense with blood vessels. Coupled with their very large surface area relative to their volume, the structures of your hands and feet are exceptionally well suited to transferring heat to your environment.

But just how important is skin temperature?

In some particularly neat experiments, participants wore ‘thermosuits’ that were perfused with water, and the temperature of the water in different body regions was manipulated by researchers. These studies unambiguously showed that it is skin temperature that most influences sleepiness, with mild skin warming facilitating sleep (2). (To be specific, skin keeps heat in the body up to about a skin temperature of about 91.4 °F (33 °C), but just a small rise in skin temperature to 95 °F (35 °C) leads to rapid heat loss (3).)

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The biological mechanisms underlying this are complex, so I’ll simplify them. A part of the brain within the hypothalamus called the preoptic area is particularly critical to integrating temperature signals with sleepiness, and experimentally warming this area in the brains of other animals makes them sleepy (4). But warm some other brain regions, and the animals become more alert. What’s key is that warming the skin promotes sleep-like signatures of electrical activity throughout the brain.

Okay, enough of the biology. How can you put this to use?

Fall Asleep Faster: Bathing and Clothing

Let’s first consider what you do before bed. It’s clear that raising your skin temperature may help you fall asleep faster. There are several ways you can do this, but a warm shower is often most practical. A 10-minute shower at about 104 °F (40 °C) shortly before bed should do the trick (5). A hot bath is another option, as is sauna use. (Sauna use generally increases skin temperature for longer than it boosts core temperature. I’m not sure there would be advantages to a sauna over the alternatives at this time though, and you might want to get out of the sauna a bit earlier than you’d finish showering.) Some experiments have used things like foot baths to specifically target the most important body parts, but you’re probably not willing to pursue this option (I’m not). When you’re done, you may want to temporarily put some warm clothes on, including socks. You could also try using a hot water bottle beneath your feet while in bed.

Next, what about your bedroom?

Fall Asleep Faster: Cool but Comfortable Bedroom

The first thing to understand is that you want to avoid extreme ambient temperatures in your bedroom, as hot and cold environments will trip your body’s temperature-defense mechanisms. There is evidence that heat loss from the skin is fastest when the ambient temperature is relatively low, so you want your bedroom to be cool. In the summer, I like using a fan aimed at my torso, for it has the added benefit of providing enough background noise to reduce the likelihood of sudden sounds disrupting my sleep.

Fall Asleep Faster: Bedding

And then there is your bedding. You may be wondering about whether heated duvets are useful. Well, they actually seem to disrupt sleep, as heated blankets may continue to add heat to the body. Bedding that better lets you control its temperature may a superior alternative. The ChilipadTM, for example, is a mattress pad that you can set the temperature of. And if you have a bed partner, (s)he will be pleased to know that you can set each side of a double bed to a different temperature.

Otherwise, go for bedding that efficiently transfers heat from your body. Sheex® mattresses, pillows, and sheets are designed to achieve this, so you may want to check out their products (or similar ones made by different companies – humanOS has no ties to Sheex).

In the future, beds that are temperature regulated using closed-loop mechanisms may be common. Much like a thermostat, such beds might activate heating and/or cooling mechanisms in response to changes in temperature under the sheets, holding the temperature under the sheets at a near-optimal level.

Final Words

Last, are there other applications of this knowledge?

You bet. Next time your trying to shirk any unwanted sleepiness, lowering the ambient temperature may boost your alertness.

I hope you’ve found this blog useful. Do share any strategies you use to fall asleep faster!

Key Takeaways

Core temperature (lower while sleeping and higher while awake) and skin temperature (higher while sleeping and lower while awake) have opposite patterns.

By accelerating heat loss, higher skin temperatures actually help reduce core temperature. This is important because it is easier to sleep when core temperature is low.

Raising skin temperature (particularly that of your extremities) may help you fall asleep faster. Try a warm shower or hot bath shortly before bed, but keep your bedroom relatively cool.