Temperature triggers sleep and influences sleep depth | Matthew Walker
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Duration and timing of light exposure are critical to our ability to sleep. But temperature plays a role, too – something easily observed in the sleep-inducing qualities of a cool bedroom. To fall asleep and stay asleep, the body's core temperature needs to drop by about 1 degree Celsius, roughly 2 to 3 degrees Fahrenheit. Cooler indoor temperatures mimic the natural pattern of cool evenings and are essential to reducing the body's core temperature. Manipulating the core body temperature through hot baths, showers, or saunas – which can have a paradoxical effect on reducing core body temperature – can have dramatic effects on our ability to fall asleep. In this clip, Dr. Matthew Walker describes the role of temperature in augmenting sleep.
Matt: [And that's on a circadian...So what is training us to our natural sleep rhythms is both temperature and light. So this comes back to our modern homes, where we go into offices, and we don't necessarily have the rising warmth of the day to activate us because] we're set at 70 degrees, and then we go home, and our thermostats are set at 70 degrees again or whatever your standard temperature is. And we don't get the thermal cue through our bodies to say, "It's time for sleep." So no wonder our sleep is worse.
And if you manipulate people's core temperature, and you can do this through showers, hot baths, you can do it through saunas, but there's a great experiment where they essentially designed what it looked like a kind of a wetsuit. And throughout the wetsuit were all of these plastic veins running through them. And the researchers were able to exquisitely control, through water at different temperatures, the temperature of any part of your body. So would I want to warm your hands and your feet? Or would I want to warm your core?
And so what they were able to do is...These people I mean talk about being stripped of your dignity, you know. You get into this like wetsuit, you zip it up, and you say, "Right, I'm off to bed." In you go. And then they would manipulate core body temperature exquisitely. And lo and behold, when they dropped the core body temperature, they were able to induce sleep quicker, and the amount of deep sleep, what we call slow-wave sleep, which is deep non-REM sleep or Stages 3 & 4, also called slow-wave sleep because of these big powerful slow brainwaves, that deep, rich non-REM sleep was increased somewhere between about 10% to 20%, which is non-trivial.
Rhonda: Wow.
Matt: Then they said, "Well," this is in sort of, you know, young healthy people, "what about people with insomnia, and people who are older." Because older people struggle with sleep and, of course, insomnia. And they were able to get even greater mileage out of the thermal manipulation with those cohorts, too. So I think it's a very interesting intervention possibility to try and augment and manipulate human sleep.
Rhonda: So do you think those things, like the chili pads that are out there, you think those can potentially help deep sleep?
Matt: Yeah. I don't know of any data from them yet. All I know is the experimental data that we've looked at, you know, with manipulation of body temperature. They did this in rats awhile back, by the way, which is they would warm their paws. And when they warm their paws, the rats fell asleep quicker and stayed asleep.
And you think, hang on a second, you just told me that you need to cool the body, but you're talking about warming them up. And this is the reason why probably saunas work, hot baths work, and showers work for the opposite reason that most people believe them to work. You think, you know, I get out of a hot bath, and I'm nice and toasty, I get into bed, and I fall asleep faster.
That's not true. What happens is that you get into the bath, you get massive vasodilation. All of the vessels open up on the surface of your skin. That draws, it almost charms, the blood out from the core of your body to the surface. And your skin and your hands and your feet especially act like these wonderful thermal radiators. And they dissipate the heat. So you get out of the bath, and your core body temperature actually plummets, and that's what you need for good sleep.
And I suspect this the reason that saunas...I mean, I think you've had experience it too.
Rhonda: Yeah, definitely. It definitely affects my sleep. And going from a sauna into, like, an ice cold shower and then doing kind of like going back and forth really really helps sleep. And I've used that also going to other countries. Getting in the sauna and going the cold shower and then back in the sauna.
Matt: And triggering it back.
Rhonda: It helps with my jet lag.
The body’s 24-hour cycles of biological, hormonal, and behavioral patterns. Circadian rhythms modulate a wide array of physiological processes, including the body’s production of hormones that regulate sleep, hunger, metabolism, and others, ultimately influencing body weight, performance, and susceptibility to disease. As much as 80 percent of gene expression in mammals is under circadian control, including genes in the brain, liver, and muscle.[1] Consequently, circadian rhythmicity may have profound implications for human healthspan.
- ^ Dkhissi-Benyahya, Ouria; Chang, Max; Mure, Ludovic S; Benegiamo, Giorgia; Panda, Satchidananda; Le, Hiep D., et al. (2018). Diurnal Transcriptome Atlas Of A Primate Across Major Neural And Peripheral Tissues Science 359, 6381.
A phase of sleep characterized by slow brain waves, heart rate, and respiration. NREM sleep occurs in four distinct stages of increasing depth leading to REM sleep. It comprises approximately 75 to 80 percent of a person’s total sleep time.
A distinct phase of sleep characterized by eye movements similar to those of wakefulness. REM sleep occurs 70 to 90 minutes after a person first falls asleep. It comprises approximately 20 to 25 percent of a person’s total sleep time and may occur several times throughout a night’s sleep. REM is thought to be involved in the process of storing memories, learning, and balancing mood. Dreams occur during REM sleep.
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