Sleep And Heat: Why Is It Hard To Sleep When It’s Hot?

Quick Summary

• Cooling is a Requirement for Sleep: To successfully fall asleep, your body must lower its core temperature. This drop in temperature is a critical signal that prepares your brain for sleep.

• Your Brain Schedules the Cooling Process: A tiny region in your brain called the suprachiasmatic nucleus (SCN), or "master clock," dictates the 24-hour rhythm of your body temperature, telling it when to cool down for the evening, facilitating sleep.

• A Warm Room Blocks Your Body's Cooling System: Your body cools itself by releasing heat through the skin, especially the hands and feet. A hot environment prevents this process, making it difficult to fall asleep and stay asleep.

• REM Sleep is Uniquely Vulnerable to Temperature Disruption: During Rapid Eye Movement (REM) sleep, your body’s ability to regulate its temperature is mostly switched off. This makes this important sleep stage highly susceptible to disruption from heat.

Sleep And Heat: Does Temperature Affect Sleep?

Tossing and turning on a warm night is a universal struggle. You feel tired, you know you want to sleep, but your body just doesn’t seem to want to come to the party.

Turns out, this isn't just a matter of feeling hot and uncomfortable; it's a fundamental conflict between your environment and your biology. The relationship between temperature and sleep is not coincidental—it's a hardwired connection that influences your ability to get quality sleep.

The difficulty of sleeping in a warm room comes from the strong, non-negotiable link between your body's sleep systems and its temperature control systems (Kräuchi, 2007; Van Someren & Deboer, 2018).

Around your usual bedtime, your body initiates a cooling process, and this process significantly assists the process of falling asleep. When this process is disrupted, it becomes harder to sleep.

So let’s take a closer look at what exactly is going on.

How Much Does Body Temperature Naturally Drop at Night?

Your core body temperature naturally drops every evening to help prepare your body for sleep. Nighttime core temperature can drop about 0.5 to 1°C (about 1 to 2 degrees Fahrenheit) below daytime levels, reaching its lowest point in the early morning hours while you sleep (around 4am give or take) (Campbell & Broughton, 1994).

So, why is the body temperature drop important to sleep?

Why Is a Cool Body Temperature Helpful for Sleep?

A cool body temperature is helpful for sleep because a slight drop in your core body temperature is one of the main physical signals that tells your brain it is time to wind down. Think of this temperature drop like the lowering the curtains at the end of a show. If the curtains stay up, your brain stays on stage, performing instead of “taking a bow” (Campbell & Broughton, 1994).

Your core body temperature (CBT), the temperature of your internal organs, naturally rises and falls throughout the day. It begins to drop in the evening, and your usual bedtime aligns perfectly with the moment your CBT is falling at its fastest rate (Campbell & Broughton, 1994). This carefully timed decline helps make any existing feelings of sleepiness more apparent, significantly reducing the time it takes you to fall asleep (Lack et al., 2008).

This process is an essential part of your natural sleep wake cycle.

But what actually controls the regulation of night time body temperature?

What Controls The Body’s Nightly Temperature Drop?

Your body's nightly cool-down schedule is controlled by the suprachiasmatic nucleus (SCN), a tiny cluster of nerve cells in the brain that functions as your master clock (also known as the master circadian clock or pacemaker).

The SCN is responsible for organizing nearly all of your body's 24-hour cycles, known as circadian rhythms, from hormone release to your sleep-wake cycle (Mitchell & Gillette, 2021).

One of the SCN's most critical jobs is to manage the daily rhythm of your core body temperature (CBT). Each evening, as part of a pre-programmed schedule, the SCN sends out signals that tell your body to begin shedding heat (Van Someren & Deboer, 2018).

This carefully timed temperature drop is one of the most powerful signals your body uses to encourage sleep. People find it easiest to fall asleep when their core body temperature is declining, a phase directly controlled by the SCN's schedule (Wright, Hull, & Czeisler, 2002).

But how does a tiny clock in the brain tell the rest of the body it's time to cool off? It uses a chemical messenger.

How Does Melatonin Affect Body Temperature?

Melatonin's primary role in the nighttime body temperature cooling process is to act as the chemical messenger that carries the SCN's "cool down" order to the rest of the body. While often called the "sleep hormone," it is more accurate to think of melatonin as the "hormone of darkness" that helps prepare your body for sleep, largely by managing its temperature.

When the SCN detects fading light in the evening, it signals the pineal gland to begin producing and releasing melatonin (Burgess & Zeitzer, 2021).

This rise in melatonin levels has a direct effect on your circulatory system. It causes vasodilation, the widening of blood vessels, particularly in your hands and feet (Kräuchi et al., 2006).

This action is not a side effect; it is one of melatonin's key functions. By triggering this heat-loss mechanism, melatonin actively helps lower your core body temperature, which in turn increases feelings of sleepiness (Van Someren & Deboer, 2018).

How Does Your Body Naturally Cool Down for Sleep?

Once the SCN gives the order and melatonin delivers the message, your body naturally cools down for sleep by moving heat from its core to the skin's surface, particularly in your hands and feet. It does this through a process called vasodilation, a temporary widening of the blood vessels near the skin (Kräuchi, 2007).

In particular, your hands and feet are uniquely designed natural radiators, helping your body to lose heat. They have a high surface-area-to-volume ratio and contain specialized blood vessel structures that allow for rapid cooling.

As your blood vessels widen, your circulatory system directs a large volume of warm blood from your core to the extensive network of small veins in these areas. This allows the heat to efficiently escape into the cooler, surrounding air (Kräuchi et al., 2000).

This is why you might notice your hands and feet feeling warm just as you start to feel sleepy—it’s a clear sign that your master clock and its hormonal messenger are successfully preparing your body for sleep.

Why Is It Hard To Sleep In A Hot Room At Night?

A hot room makes sleeping harder because the high room temperature blocks your body's ability to cool via releasing heat into the environment. The body’s entire cooling system depends on a simple principle of physics: heat moves from a warmer object to a cooler one.

If the air in your bedroom is too warm, there isn't enough of a temperature difference between your skin and the air. Your body simply cannot get rid of the heat effectively.

When this happens, your core temperature remains high, the "go to sleep" signal is weakened, and you lie awake feeling "stuck" between wake and sleep.

Your body is trying to follow its internal clock, but the external environment is preventing a critical step in the process.

So it’s a natural, understandable (frustrating) situation.

But is it all about falling asleep? What about during the rest of the night? Does heat affect sleep throughout the night?

Does Heat Disrupt Sleep Throughout the Night?

Heat continues to disrupt sleep all night long in two main ways:
1) it causes more frequent awakenings (aka “fragmented sleep”), and
2) it changes the natural pattern of your sleep.

Even after you’ve fallen asleep, your body remains sensitive to the room's temperature. This forces your brain to constantly choose between staying asleep and dealing with the thermal stress, a battle that sleep often loses.

What is 'Sleep Fragmentation' and How Can Heat Cause It?

Sleep fragmentation is an increase in brief awakenings or shifts to lighter, less restful sleep stages, and heat is a primary cause. Studies show that sleeping in a warm environment leads to this kind of fragmented sleep (Buguet, 2007; Van Someren & Deboer, 2018).

This is disruptive no matter the cause, making your sleep less refreshing and leading to next-day consequences like poor concentration, memory lapses, and irritability (Carskadon & Dement, 1977).

How Does Heat Affect The Different Stages of Sleep?

Heat especially reduces the amount of time you spend in deeper, more restorative sleep stages. Research has consistently shown that sleeping in warm conditions decreases the duration of NREM sleep (Muzet et al., 1984).

NREM sleep includes the deep sleep that is critical for physical repair, hormone regulation, and immune system function.

However, the sleep stage most affected by heat is REM sleep, which is vital for mental restoration.

Why Is REM Sleep Disrupted By Heat?

REM sleep, the stage of sleep associated with vivid dreaming, is uniquely vulnerable to heat because your body’s main temperature-control systems are mostly switched off during this stage. While you can normally sweat or shiver to regulate temperature, these responses are strongly blocked during REM sleep (Van Someren & Deboer, 2018).

During the REM sleep stage, your body temporarily loses much of its ability to control its own temperature, making it much more dependent on the temperature of your bedroom (Glotzbach & Heller, 1976).

It's as if your brain decides that the intense work of dreaming and memory consolidation is so important that it temporarily diverts resources away from other tasks, including managing body temperature. If your room is too warm, your body cannot sustain this vulnerable state. The stress from the heat becomes too great, and your brain will shorten or exit REM sleep to regain temperature control. This results in a significant loss of this critical stage for learning and emotional processing (Horne, 1992).

So if normal sleepers can experience sleep difficulties on hot nights, what about those with sleep disorders like insomnia Are hot nights even worse for insomnia sufferers?

Is It Even Harder For People With Insomnia To Sleep When It's Hot?

Yes, it can be particularly challenging for individuals with sleep-onset insomnia, a condition characterized by difficulty falling asleep. Research suggests that some people with this condition may already have a less efficient heat-loss response at bedtime (van den Heuvel et al., 2006). A warm environment can worsen this underlying issue, making it even more difficult to achieve the necessary core body temperature drop to initiate sleep (Kräuchi et al., 2008).

But is it all about external heat?

Can your own body’s heating processes disrupt sleep?

Can Internally Generated Heat Also Disrupt Sleep?

Yes, internally generated heat from events like menopausal hot flushes (USA English: hot flashes) can disrupt sleep just as much as an overly warm room. These events are not just a feeling of warmth; they are powerful episodes related to temperature control that trigger the body's emergency cooling mechanisms. It's like having a personal, internal heatwave that strikes without warning.

How exactly do hot flushes wake you up?

Hot flashes wake you up by triggering a sudden, urgent need to cool down, which activates your body's "wake up" systems. A hot flash is caused by a malfunction in the brain's internal thermostat, making it suddenly believe the body is overheating. This triggers an intense wave of vasodilation and sweating designed to rapidly cool the body (Freedman, 2005). This physiological alarm is often strong enough to override the drive for sleep. Research confirms this, with one study finding that 69% of objectively measured nocturnal hot flashes were directly associated with an awakening (de Zambotti et al., 2014).

Does it matter which sleep stage a hot flush occurs in?

Yes, the sleep stage in which a hot flash occurs has a significant impact on whether it will cause an awakening. Hot flashes are much more likely to wake a person up if they happen during NREM sleep. Interestingly, if a hot flash occurs during the brain's intense activity in REM sleep, it is less likely to trigger a full awakening (Baker et al., 2019; Freedman & Roehrs, 2006). This suggests the brain's priorities shift depending on the sleep stage it's in.

What Are the Practical Takeaways for Better Sleep in the Heat?

Understanding the science behind temperature and sleep allows you to take targeted steps to create a better sleep environment.

• Create a Cool Bedroom: The most direct solution is to lower your room's temperature. While the ideal temperature varies, the recommended range is suprisingly cool, between 15-19°C (60-67°F). Use air conditioning, fans, or open windows to achieve this.

• Take a Warm Bath Before Bed: A warm bath or shower an hour or two before bed can aid sleep. The warm water draws heat from your core to your skin. When you get out, the rapid evaporation of water from your skin creates a "rebound effect," causing your core body temperature to drop, mimicking the body’s natural process of signalling evening wind down.

• Choose Breathable Bedding: Use sheets and pajamas made from natural, breathable fibers like cotton, linen, or bamboo. These materials allow air to circulate and wick moisture away from your skin. Synthetic fibers like polyester tend to trap heat and moisture, creating a personal sauna effect.

• Keep Your Hands and Feet Uncovered: Since your hands and feet are your primary heat-release zones, think of them as your personal vents. Leaving them outside the covers can help your body continue to regulate its temperature throughout the night.

• Discuss Internal Heat with a Professional: If you experience sleep disruption from nocturnal hot flushes, it's important to treat it as a medical issue. Speak with a healthcare provider to discuss management strategies and treatments that can improve your sleep quality during menopause.

• Discuss Sleep Disorders with a Professional: Hot nights may make the symptoms of a sleep disorder worse, but resolving the bedroom heat issue won’t resolve a sleep disorder like insomnia or obstructive sleep apnea. Talk to a healthcare provider to discover the recommended treatment for insomnia, circadian rhythm disorders, and science-backed treatments for other sleep disorders.

You can also talk to a NZ sleep clinic like The Better Sleep Clinic for sleep help. Whether it’s an Auckland sleep clinic, Wellington sleep clinic, Christchurch sleep clinic, Hamilton sleep clinic or anywhere in NZ, we can help.

Ask for a free chat below or book an assessment (no referral required) and get started addressing your sleep problems today.

Frequently Asked Questions: Sleep, Heat, And Temperature

Q1: Why is a drop in body temperature important for sleep?

A1: A drop in body temperature is important for sleep because it is one of the main physical signals that tells your brain it is time to wind down (Campbell & Broughton, 1994). This carefully timed decline in your core body temperature helps make you feel sleepy and significantly reduces the time it takes you to fall asleep (Lack et al., 2008).

Q2: What part of the brain controls the body's nightly temperature drop?

A2: The body's nightly temperature drop is controlled by the suprachiasmatic nucleus (SCN), a tiny cluster of nerve cells in the brain that functions as your master clock (aka the “body clock”) (Mitchell & Gillette, 2021). The SCN manages the daily rhythm of your core body temperature and sends out pre-programmed signals each evening that tell your body to begin shedding heat (Van Someren & Deboer, 2018).

Q3: How does melatonin help the body cool down at night?

A3: Melatonin acts as the chemical messenger that carries the "cool down" order from the SCN (body clock) to the rest of the body. When melatonin is released in the evening, it causes vasodilation - the widening of blood vessels in your hands and feet - which is a key mechanism for losing heat and lowering your core body temperature (Kräuchi et al., 2006; Van Someren & Deboer, 2018).

Q4: Why exactly is it so difficult to sleep in a hot room?

A4: It is difficult to sleep in a hot room because the high temperature physically blocks your body's ability to release heat. Your body needs to transfer heat to a cooler environment to lower its temperature. If the room is too warm, there isn't enough of a temperature difference for this to happen, your core temperature stays high, and the powerful "go to sleep" signal is weakened (Kräuchi, 2007).

Q5: Why is REM sleep especially vulnerable to being disrupted by heat?

A5: REM sleep is uniquely vulnerable to heat because your body’s main temperature-control systems, like sweating and shivering, are mostly switched off during this sleep stage (Van Someren & Deboer, 2018). This makes your body temperature much more dependent on the room's temperature. If it gets too warm in the room, your brain will shorten or exit the REM stage to regain control over its temperature (Horne, 1992).

Q6: Is it even harder for people with insomnia to sleep when it's hot?

A6: Yes, it can be particularly challenging for people with sleep-onset insomnia. Research suggests their bodies may not cool down as effectively at bedtime to begin with, so a warm room creates a "double barrier" that makes it even more difficult for them to achieve the necessary temperature drop to fall asleep (van den Heuvel et al., 2006; Kräuchi et al., 2008).

Q7: How do internal hot flushes wake a person up?

A7: Hot flushes wake you up by triggering a sudden, urgent need to cool down that activates your body's arousal systems. This is caused by a malfunction in the brain's internal thermostat, leading to intense vasodilation and sweating (Freedman, 2005). This physiological alarm is often strong enough to override the drive for sleep, with one study finding that 69% of nocturnal hot flashes were directly associated with an awakening (de Zambotti et al., 2014).

References

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Written By The Better Sleep Clinic

Reviewed By Dan Ford, Sleep Psychologist