© Getty Images/KJNThe Shower
For many people, a shower or bath in the evening is a way to wash off the day, both literally and figuratively.

Besides signaling your body that the day is over and it's time to wind down, water immersion can affect your core body temperature. And because core body temperature plays a part in our sleep wake cycle โ€” we naturally experience a drop of around 2-3 degrees Fahrenheit in the evening and during sleep - researchers have begun to study how influencing this natural regulation may affect sleep quality.

So, the question becomes: Which is better, a warm or hot bath or shower before bed, or a cold one?

Here's what the science shows:

Benefits of a hot shower before bed

There's actually some good science, and a growing body of research, around how taking a hot shower or bath before bed can promote better sleep by augmenting our body's natural evening cool-down. Here's how that works.

Throughout the day, your core body temperature naturally fluctuates in accordance with your circadian rhythm, which controls your sleep-wake cycle along with lots of other biological functions. Your body temperature is one of the cues your brain relies on to determine whether it's time to be asleep or awake.

A cooler body temperature signals that sleep time is approaching, and correlates with an increase in melatonin.

Although it seems counterintuitive, taking a hot bath or shower at night aids this natural thermoregulation process and cools you off. The hot water heats up your body, bringing blood flow to the surface, and then when you get out of the shower your body heat quickly escapes through your hands and feet. This rapid cool-down of your core temperature has been shown to help you fall asleep faster, as well as promote deeper sleep. Scientists guess that one reason is that the quick temperature drop encourages rapid melatonin production.

A 2019 meta analysis found that taking a shower or bath for at least 10 minutes, where the water was between 104 and 108 degrees Fahrenheit, one to two hours before bedtime, improved participants' sleep quality more than those who didn't take one. A warm/hot shower may also improve sleep by relaxing you, physically and mentally. When you're more relaxed it's easier to fall asleep.

The National Heart, Lung, and Blood Institute recommends taking a warm shower or bath before bed as a way to lower blood pressure as well as improve sleep.

Benefits of a cold shower before bed

If you're a biohacker or overachiever you might assume that a cold shower would speed up the body's cooling process and with it melatonin production, and sleepiness. You may have heard of "the Iceman" Wim Hof, the eccentric Dutchman and wellness guru who has inspired extreme athletes, tech entrepreneurs, and a number of mere mortals with his preternatural ability to withstand cold exposure. He claims that the many health benefits of ice baths include fat loss, reduced inflammation, a fortified immune system, balanced hormone levels...and better sleep quality.

Though scientists have become increasingly intrigued by Hof, those who've studied cold immersion as a way to optimize sleep have thus far had mixed results (though there's no shortage of testimonials as to how a cold shower or bath before bed can knock you out).

According to David Randall, author of Dreamland: Adventures in the Strange Science of Sleep, a study by researchers in Lille, a city in northeastern France, found that subjects fell asleep faster and had a better overall quality of sleep following behaviors that cooled the body, such as taking a cold shower right before bed. Another study found that athletes who immersed themselves in cold water for 10 minutes after evening exercise experienced a drop in core body temperature, fewer nighttime arousals, and a greater proportion of deep sleep within the first three hours of sleep.

But not all research has been as promising. One study of male cyclists found that being immersed in cold water after evening exercise did not affect sleep architecture. A similar study of young soccer players found that cold water immersion after an evening training session also did not impact sleep quality. (Although athletes or exercisers may find that cold showers help reduce muscle stiffness, which could very well make falling asleep easier.)

One reason a cold plunge might not improve sleep is that it's known to activate the sympathetic nervous system and increase blood levels of beta-endorphin and noradrenaline as well as increase synaptic release of noradrenaline in the brain.

One study on cold exposure found that the body releases cortisol when exposed to the cold, while another found that increased levels of cortisol at night may lead to poor sleep. This could suggest that taking a cold shower before bed may be less beneficial for a good night's sleep, and better saved for shocking yourself awake on a sluggish morning.

On the other hand, enduring a blast of cold water will definitely keep you focused on the present moment (brrrrrr!) and likely take your mind off the day's worries so that when your head hits the pillow it will be free of stress โ€” that thing that often keeps us awake.

Everything in moderation

According to some experts, if you're a fan of showers at extreme ends of the temperature spectrum, you may want to rethink the habit. Rachel Salas, MD, a sleep neurologist at the Johns Hopkins Center for Sleep and Wellness, told The Washington Post that taking a really hot or really cold shower before bed could lead to sleep problems, since "what you're doing is you're making your body's temperature so different from your baseline," she said. Salas recommends taking a more moderate temperature bath or shower if you're planning to head to bed soon after.

Extremely hot or cold showers are also not recommended if you have a heart condition. Research also shows that hot water may cause more dramatic changes to blood pressure in older adults.

So, which is it? Hot or cold for better sleep?

Like most things, it depends on you and your body. For the record, baths and showers work equally well. And author/podcaster/wellness guru Tim Ferris has had success with both.

The benefits of hydrotherapy have been known for centuries, since the ancient Greeks and then the Romans turned to thermal cures for their healing properties. So, experiment. Test out both cold and hot showers to see which one works best for you.

You likely can't go wrong with a warm (or hot) shower about 90 minutes before bedtime. Plus, there's more science to back up the positive effects of warm or hot baths or showers on sleep.

In terms of health benefits, taking a cold shower for up to 5 minutes 2 to 3 times per week has been shown to relieve symptoms of depression in one clinical trial. The cold water sends many electrical impulses to your brain and triggers the release of mood-boosting hormones including endorphins, which can elicit feelings of optimism, wellbeing, and feeling more in control. Something we could all use more of these days.

If you want to try a cold shower, start slow. Hof recommends you start with a warm shower and end with a few seconds of cold water, building up to a minute or more. Just don't do it right after a meal, when your body is busy digesting your food.

Bottom line: There aren't really studies comparing hot and cold showers head-to-head. Everyone has a different response to water temperature, so go with your personal preference. Whichever temperature you choose, taking an evening shower has many benefits ... one of which is simply being clean when you get into bed.

Reviewed by Jenna Gress Smith, PhD