Should You Warm Up or Cool Down for a Good Night’s Sleep?

You probably already know that being cool is best for sleep. Not only is a cooler environment most conducive for drifting off, but your body temperature also actually declines in preparation. But did you know that both warm baths and ice baths can enhance this process? The secret is in the timing. Knowing how your body temperature fluctuates over the course of your waking hours can help you be strategic about how to augment that cycle.

Before you run that warm bath (or brace yourself for a cold plunge), read on to learn more about the connection between internal temperature and sleep.

To understand how core body temperature changes over a 24-hour period, we need to go back to our biological clock—the internal timekeeper that helps govern the sleep-wake cycle and is kept in sync by our exposure to light and dark. The master clock in our brain, the suprachiasmatic nucleus (SCN), functions as the overall timekeeper, helping regulate the many other internal clocks within the body that govern various physiological functions. One is body temperature, which rises and falls with our sleep-wake cycle.

This fluctuation isn’t much—only about one degree—but it occurs in a set wave pattern, rising over the course of the day to peak in the afternoon, then descending to its lowest point in the hours before we wake before beginning its daily ascent again.

While the nightly drop in your core temperature is taking place, there’s a second temperature change that’s also happening: Your distal temperature (the temperature in your hands and feet) is actually increasing as the blood vessels there dilate. This boost in blood flow to your hands and feet draws heat away from your core, which helps facilitate the drop in core temperature that’s linked to the onset of sleep. In fact, past studies measuring participants’ nightly core temperature decline have found that the point when they felt sleepy enough to choose to go to bed actually corresponded to when their temperature was in its steepest decline.

The difference between these two temperatures (core body temperature and distal temperature) can be as much as 2.7 degrees Fahrenheit as heat is channeled to your hands and feet before it starts to level off as your core temperature declines. Then, about two hours after you fall asleep, your core temperature reaches its lowest point before gently starting to rise again.

Generally, this nightly distal warming occurs in tandem with our actions as we prepare for bed: We’re moving around less, and our muscles are less active, helping set the stage for this increased blood flow to be directed to our hands and feet.

As bedtime approaches, do you ever find yourself wanting to curl up under a blanket? That craving for coziness may be biologically based. It’s a behavior known as nesting, and it actually helps increase your distal temperature in preparation for sleep. By snuggling and creating a mini cocoon of warmth, you’re helping increase warmth (and vasodilation) in your hands and feet, which can jump-start this process of cooling your core temperature.

You can boost this bedtime process of “warming up to cool down” through other actions, too, such as soaking in a warm bath or taking a shower. By doing so, “you’re temporarily bumping your body temperature up,” explains Dr. Chris Winter, neurologist and author of “The Rested Child.” A warm shower doesn’t actually warm you up. “When you hop out of the shower, and you’re cooling down, it’s kind of a little trick to create a quick temperature drop.”

As for when you should hop in the tub, research shows that doing so one to three hours before bedtime is your best bet if you want to fall asleep faster. (Any closer to bedtime, you won’t have enough cooldown buffer afterward.)

Sipping a steaming mug of herbal tea, like chamomile or lavender, or a glass of warm milk before bed can have a similar effect on your core temperature, says Winter. “It warms you up a bit,” he explains, “and then as your temperature starts to drop again, it’s a trigger for feeling sleepy.” Just be sure to avoid caffeine, which temporarily blocks the brain’s receptors for adenosine and masks sleepiness.

Augmenting your body’s pre-bedtime cooling process isn’t the only way that sipping a warm beverage or slipping into a steamy bath can help your sleep. Both can also be part of a regular wind-down routine, a nightly relaxing ritual that helps you mentally unwind and prepare for bed.

Once you know when these temperature peaks and valleys happen, you can take steps during the day to magnify this curve, Winter explains. Depending on the timing, this can mean supplementing with a sauna or hot tub for heat therapy or an ice bath for cold therapy.

To amplify your body’s peak core temperature, which occurs around 4 p.m., late afternoon would be the optimal time to hop in the sauna, Winter says. “Now your temperature is higher than it would have been ordinarily at 4 p.m.,” he explains, “so it’s starting to fall from a higher place.”

Similarly, a cold plunge can be used early in the day to accentuate the low point of the curve. “Go out and jump in your cold plunge or take a cold shower first thing in the morning,” Winter says, “so that low nadir of body temperature becomes even lower.”

What if you’ve ended up elevating your temperature at the “wrong” time to promote sleep?

This can happen if you work out in the late afternoon or evening—a common scenario if you exercise after the end of the workday.

If you’ve worked out around dinnertime, Winter points out, “you’ve created a problem with your body temperature curve by raising it every night at 6 p.m. when it should have been on its way down since 4 p.m.”

The best way to counteract this (other than shifting the timing of exercise): a cold plunge or shower. “They’re great to do immediately after you exercise to get your body temperature back on the path of going down and getting ready for sleep,” says Winter, “so it’s back on the schedule it should have been on had you not exercised.”

Many athletes face similar timing challenges, given the realities of evening games, he notes. “After the game is over and it’s time to go to bed, and your temperature is still very high, it can make sleep difficult,” he says.

In this scenario, a cold plunge can help reverse the situation. In one study, endurance athletes completed a high-intensity run at 6 p.m. and then immersed themselves in cold water for 10 minutes. Some of the athletes stood upright in hip-deep water so they were only partially submerged, while others wore scuba gear so they could be completely underwater.

Fortunately, you don’t need scuba gear to reap the benefits of cold-water immersion: Both groups in the study showed a decline in core temperature, but it was more pronounced in those fully submerged.

“Just hop into your hot tub when it’s cold,” says Winter, “or fill your bathtub with water and ice.”

The goal, he explains, is to minimize the negative effects of the exercise-induced temperature rise and coax your core temperature back to where it would have been. Otherwise, you run the risk of phase-delaying your sleep, just as you would by being exposed to bright light in the evening.

Cold therapy can also help promote sleep by reducing pain and inflammation, which in turn helps you sleep better.

Finally, as you think about cooling down and warming up to help your sleep, don’t forget your household thermostat. In addition to helping keep your sleeping environment cool (which helps you fall and stay asleep), your thermostat can be programmed to “follow that natural temperature curve,” says Winter. He recommends gradually lowering the temperature as the curve descends in the early evening, then gently raising it again an hour or so before it’s time to wake up.

While the best temperature for sleep is generally between 60 and 68 degrees Fahrenheit, keep in mind that it can vary based on age and related factors.

By middle age, some adults may prefer a sleeping temperature on the cooler end of the recommended range to counteract the effects of medications or other health realities.

Up to 80% of women experience night sweats, which can begin during perimenopause and persist for several years afterward, disrupting sleep.

Additionally, obstructive sleep apnea, which causes multiple wakings throughout the night, becomes more common as we age. Research shows that keeping the bedroom at a cool 60 degrees Fahrenheit can help counteract sleep-apnea-related disruptions, including reducing time spent awake and boosting overall sleep time.

“Temperature is a modifiable variable, just like light is,” Winter sums up. “We have a lot of conversations around light as it affects sleep, but I think temperature sometimes gets forgotten.”