Utilizing different forms of heat and cold for health benefits has become very trendy in recent years, especially the use of the sauna and cold plunge. But is the hype warranted? My short answer would be yes to sauna, maybe for cold plunge. Like all major health questions, though, this answer requires some nuance, and I believe a lot of the benefits of either may be mediated through the time of day in which they are done.
The emerging field of chronobiology highlights this importance of timing various events or activities across our day. This area of study explores the optimization of our circadian rhythm, which is just to say how our body regulates our sleep-wake cycle and things like energy levels appropriately throughout the day. Not only can fine-tuning our circadian rhythms help us to feel significantly better in daily life, ready to take on the day in the mornings yet equally restful at night, but it can also have significant impacts on long-term health and disease risk. This is largely accomplished through improvements in sleep, as well as balance of our sympathetic (“fight or flight”) and parasympathetic (“rest and digest”) nervous system activity. In a general sense, this nervous system balance can be assessed via outputs like heart rate variability (HRV), leading many (including myself) to seek health wearable devices that assess this and other circadian metrics.
One of the metrics that most clearly tracks with the time of day is body temperature. Starting at midnight, our body temperature should be just about at its lowest. In the hours closer to wake-up time, this temperature should actually start to rise as we exit deeper sleep stages in favor of rapid eye movement (REM)-heavy sleep stages, and prepare ourselves for wakefulness. It will continue to rise and hit its peak in the afternoon, then start its drop down for bedtime. This is one reason why it’s so important to go to sleep and wake up at the same time every day: our body adjusts its timing of all of this to our typical 24-hour schedule. If you go to sleep or wake up hours apart, among other things, your temperature will be completely inappropriate for what you are trying to do. It can be pretty miserable to try and sleep when you are too hot, or try to get up and start the day when your body is still in the cold, deep darkness of sleep.
This is where hot and cold modalities come in, as different components that could potentially help regulate this circadian rhythm. While I maintain that the most important principle for this is probably still that sleep regularity in addition to light and sunlight exposure, it appears to help to use hot and cold to adjust our body temperature in accordance with what it should be, and reinforce that 24-hour schedule. However, the optimal pattern might not be what you think, given that we want to have a high core body temperature in the mornings, and low in the evenings.
Paradoxically, the thought is actually that it would be optimal to do cold exposure in the mornings, and heat exposure in the evenings. This is for a few different reasons, which are more or less specific to the modality. Looking at cold first, pretty much all the common forms of cold exposure, such as a cold plunge or cold shower, are effective at decreasing peripheral body temperature (meaning the temperature out away from our core, like in our extremities, for example). Our body actually then compensates for this drop in temperature by raising core body temperature to protect our most important organs, like the brain and central nervous system, heart, etc. Additionally, this response to significant cold exposure triggers a sympathetic nervous system response of catecholamine hormones like epinephrine and norepinephrine. This time of day upon waking is when we want to be just about at our peak “wakefulness” and alertness with this sympathetic tone, as we have the rest of our day to go. This is one reason why caffeine is also good to have as early as possible in the morning, as it drives sympathetic tone.
We of course want this sympathetic tone to fade out later in the day and favor the restfulness of our parasympathetic nervous system to prepare us for bed, for which we can use heat such as sauna or a hot shower. Sauna actually does quite an effective job at raising core body temperature, possibly exhibiting its benefits by almost simulating the body’s response to cardiovascular exercise. This rise in core body temperature is again followed by compensatory efforts from the body to cool down, leading to a steep drop in core body temperature upon exiting the sauna. You may recall that falling asleep requires a significant drop in our core body temperature, and a sauna session just before bed can help so much to produce this that it can feel almost sedative when your body hits the sheets. The evidence is much stronger–almost unambiguous–for the efficacy of sauna more than cold plunge on not just cardiovascular outcomes, but all-cause mortality. It is unclear the mechanism of benefit from sauna, or the degree to which it is mediated through positive effects on sleep and circadian rhythm, but I would not doubt that this is playing a major role in its upside.
In summary, while we want to reinforce our circadian rhythm by encouraging appropriate body temperature fluctuation across the day, it’s actually a little counterintuitive in the way we want to do it. In the mornings, we want to cool ourselves down with a cold plunge or cold shower, in order to actually warm ourselves up and promote wakefulness, alertness, and arousal. Come evening time, we can warm up, ideally with a dry sauna but even with a hot shower or hot tub, in order to rapidly cool down and lull ourselves to sleep. This use of temperature should probably be viewed as supplemental to strong forces like sleep regularity and morning-evening sunlight viewing in order to optimally tune our circadian rhythm. When done properly, this can help to modulate energy levels accordingly, promote healthy restorative sleep, and prevent a host of chronic health issues from arising.
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