Hot & Cold Showers: How bathing affects our sleep?

Have you ever thought whether you would like to have a cold shower or a hot shower? You might think, “Does it really matter… We just bath to get fresh-up isn’t it!” Well, that’s partially true, but it has more to it. The temperature of water you shower has some interesting effects. In short, it’s as follows:

Understanding how our sleep cycle is affected based on what type of shower we have and when we have it.

Interesting, isn’t it! But how does it happen?

Before we can understand that, we need some primers.

1. Core body temperature rhythm

Have you ever noticed that the thermometer readings taken at different times of the day are different? If so, don’t worry, it’s not entirely your thermometer’s fault :)

Our body doesn’t stay at the same temperature through out the day. It closely follows the circadian rhythm (basically, a sleep-wake, day-night cycle.)

See how a normal body temperature fluctuates through out the day. Looks similar to a Sine wave (Source: https://ouraring.com/blog/natural-body-temperature/ )

For an average human, the body temperature raises early in the morning, and continues to steep up till late evening. The moment it plummets, it’s an indirect signal for us to go to bed in a couple of hours. So, we will probably go to sleep a couple of hours after the core body temperature starts plummeting. The same way, we will wake up a couple of hours after the core body temperature begins to raise (called as Temperature minimum, i.e the temperature at around 4AM)

2. Effect of bath on core body temperature

Now, suppose you had a hot shower. Will your body temperature raise or fall?

Well, think of it this way. An external agent (hot shower) is trying to change your core body temperature. And it’s a rapid change. So, even though, there’s a momentary raise in temperature, your core body temperature goes in opposite direction i.e it falls down. (Basically, fighting the external agent)

3. The final picture

Now, combine the above pieces, and ask yourself this question, “What happens if I have a hot shower in the evening?”

Well, irrespective of whether it’s evening or not, a hot shower will reduce the core body temperature. So, there’s a drop in temperature in late evening. And that corresponds to … “a signal for early sleep”

Tried to plot the change in core-body temperature in different scenarios. Top: Cold shower, Bottom: Hot Shower. Left: Shower in the morning, Right: Shower in the evening. Pardon me for informal representation :) (Viewed differently in browser vs mobile)

In short:

Cold shower in morning i.e increase in core temperature → Phase advances our temperature rhythm→ Helps in sleep early → Helps in waking up early the next day

Cold shower in evening i.e increase in core temperature → Phase delays our temperature rhythm → Helps to sleep late → Difficult for us to wake up early the next day

Hot shower in morning i.e decrease in core temperature → Phase delays our temperature rhythm → Helps to sleep late → Difficult for us to wake up early the next day

Hot shower in evening i.e decrease in core temperature → Phase advances our temperature rhythm → Helps to sleep early → Helps in waking up early next day

So, next time you have difficulty in waking up early, you know what to do: Cold shower in morning and hot shower in evening :)

Argh!! I’ve to tremble in cold shower early in the morning then!!?

The story isn’t over. It really depends on what you want to achieve…

Shivering/trembling in cold shower → Accelerates fat loss

Resisting the trembles in cold shower → Improves stress tolerance/stress inoculation

When we shiver in cold bath, we release succinate (a compound) which acts on brown fat and accelerates the metabolism (thermogenesis) — More on this in Huberman Podcast-Episode 21 (maybe a future blog)

When we resist the shivers, we are inherently training our mind to not panic/wander around during tough times and try to stay calm i.e an indirect way of simulating real-world stress and getting inoculation.

So, if you want to improve your stress levels, resist the shiver. If you want to improve metabolism/fat loss, just shiver away.

Hmmm..interesting..what about food and exercise timings?

Well, that covers an entire blog. But an overview can be given as:

Eating and Exercise both increases core body temperature. So,

Eating and/or exercise in the morning → Increase in core body temperature → Phase advances → Helps to sleep early → Helps to wake up early

Eating and/or exercise in the evening → Increase in core body temperature → Phase delays → Helps to sleep late → Difficult for us to wake up early the next day

Disclaimers

When it comes about health, it’s obvious to have disclaimers isn’t it! So, here it goes:

1. I am not an expert in this field, so don’t take the advice for granted. This blog is inspired from Huberman Podcast-Episode 3, an excellent podcast, highly recommend for everyone who wants to learn about science and science based tools for better life.
2. What I explained above isn’t the full picture (there’s still a lot going under-the-hood) and isn’t 100% accurate. For eg, it’s not super simple to control the sleep cycle by shower/core temperature. While they help, there are other important factors like light (super important), exercise, eating habits.
3. There’s no one-size-fits-all principle in biology. For eg: I experienced exercising in the evening helps me to go to bed early. So, there’re a lot of other factors playing around.

P.S: I am trying to make notes for this podcast and some occasional (condensed) blog posts. The detailed notes can be referred here. This notes is not a replacement to Huberman podcast and it’s best used as a reference material. Once again, do checkout the podcast as it’s highly packed with lot of good stuff. It’s available in Youtube, Spotify, Apple Podcast etc;