Do we get hotter when we sleep? This is a common question many people have wondered about at some point, especially after waking up feeling overly warm or sweating during the night. The short answer is yes, our body temperature does increase slightly when we sleep. However, the reasons for this increase are complex and have to do with the intricate changes our bodies go through during different stages of sleep.
In this article, we’ll explore the science behind why we get hotter when sleeping. We’ll look at how body temperature is regulated, the changes that occur during the different sleep cycles, and some theories as to why our temperature goes up at night. We’ll also provide tips on how to sleep cooler if night sweats are an issue. Read on to learn more about this fascinating aspect of human physiology and sleep.
Regulating Body Temperature
Our bodies work hard to maintain a consistent internal temperature of approximately 98.6°F (37°C). This process of keeping our temperature in a narrow range is called thermoregulation. The hypothalamus, a small region of the brain, acts as the body’s thermostat by receiving input from temperature receptors in the skin and core.
When the hypothalamus senses the core temperature is rising above the set point, it initiates heat loss mechanisms like vasodilation (widening of blood vessels to increase blood flow to the skin) and sweating. These physiological changes allow excess heat to dissipate from the body. On the other hand, if the core temperature drops too low, the hypothalamus triggers mechanisms like vasoconstriction (narrowing of blood vessels to decrease blood flow to the skin) and shivering to conserve or generate more heat.
Why Body Temperature Fluctuates
While the hypothalamus tries to keep our temperature steady, certain factors can cause fluctuations above or below the 98.6°F set point:
- Exercise and physical activity can raise body temperature.
- Environmental temperatures influence body temperature – hot environments raise it, cold environments lower it.
- Infections and illness often cause fever, which is an elevated body temperature.
- The menstrual cycle causes a slight temperature increase after ovulation.
- Pregnancy leads to a higher basal body temperature.
- Emotions and stress can increase temperature due to activation of the sympathetic nervous system.
Interestingly, body temperature also fluctuates predictably throughout the 24-hour day. Temperature is lowest around 4-5am and highest between 4-6pm. This circadian rhythm of temperature is influenced by our sleep-wake cycles.
This brings us to the question of why body temperature increases during sleep…
Body Temperature and Sleep Cycles
To understand why we get hotter when sleeping, we need to first understand how body temperature is affected by the different stages and cycles of sleep.
There are two main phases of sleep:
- Non-REM (NREM) Sleep – consists of three progressively deeper stages (N1, N2, and N3). During NREM sleep, body temperature decreases by about 1-2°F from normal waking temperature.
- REM Sleep – is when rapid eye movement and dreaming occurs. Brain activity increases but muscles relax. Core body temperature noticeably increases during REM sleep, returning to near-normal waking levels.
We cycle through these non-REM and REM stages multiple times throughout the night. The first REM period usually occurs about 70-90 minutes after falling asleep. The REM cycles get longer through the night with the final one lasting about an hour.
Sleep Stages and Body Temperature
Here is a table summarizing how core body temperature changes during the different sleep stages:
| Sleep Stage | Characteristics | Body Temperature Change |
|---|---|---|
| Light Sleep (N1) | Transition from wake to sleep | Slight decrease |
| Moderate Sleep (N2) | Still easily awakened | Further decrease |
| Deep Sleep (N3) | Difficult to awaken | Deepest decrease |
| REM Sleep | Dreaming and muscle paralysis | Rises to near-normal waking temp |
So in summary, body temperature decreases during NREM sleep but rebounds back up during REM sleep. The cycling between these two phases is why we get hotter when sleeping. The temporary rises in temperature occur during the REM periods scattered through the night.
Theories on Temperature Increase During Sleep
Now that we know we get hotter primarily during REM sleep, the next question is why does this temperature increase happen? Scientists don’t have a definitive answer yet, but here are some of the theories:
Brain Activity
One theory is that the rise in brain activity during REM sleep generates more metabolic heat. Positron emission tomography (PET) scans show brain metabolism increases during REM sleep. This increased neural activity could cause greater heat production. However, some experts estimate this only accounts for about 13% of the nighttime temperature increase.
Thermoregulation Changes
Another hypothesis is that the thermoregulation process itself is altered during REM sleep. The threshold body temperature set point may be elevated and the heat loss mechanisms may be temporarily disabled or work less efficiently. For example, research shows the threshold for sweating is higher during REM sleep. These types of thermoregulatory changes could prevent heat dissipation and cause a rise in core temperature.
Endocrine Changes
A third possibility is that endocrine changes during REM sleep affect temperature regulation. For example, secretion of melatonin (a hormone that helps control the sleep-wake cycle) is inhibited during REM sleep. Since melatonin helps lower body temperature, this reduction may contribute to the temperature increase. Alterations in other hormones like prolactin, thyrotropin, and growth hormone may also play a role.
Circadian Rhythm
Some researchers think the nightly temperature fluctuation is linked to the circadian rhythm of core body temperature. The natural rise and fall through the 24-hour cycle could cause the increase during REM sleep periods, which tend to occur more frequently closer to morning when circadian temperatures are rising. However, more research is needed to fully understand this relationship.
The exact physiological triggers resulting in higher temperatures during REM sleep are still under debate. The increase is likely due to a combination of the factors described. But the cycling pattern clearly demonstrates that body heat generation and loss are fundamentally different during sleep compared to wakefulness.
Tips for Sleeping Cooler
For most people, the hot flashes that can accompany REM sleep are a minor annoyance. However, excessive night sweats can affect sleep quality and daytime energy. Here are some tips for sleeping cooler if nighttime overheating is an issue:
Adjust Room Temperature
– Keep bedroom between 60-67°F (15-19°C)
Use Breathable Bedding
– Cotton sheets and lightweight blankets allow more air circulation and moisture wicking
Limit Bedding Layers
– Use fewer blankets to prevent heat trapping
Wear Light, Breathable Pajamas
– Avoid heavy or insulative fabrics
Take a Cool Shower Before Bed
– Lowers core body temperature before getting in bed
Stay Hydrated During the Day
– Prevent dehydration which can raise risk of night sweats
Manage Room Humidity
– Use dehumidifier to keep humidity below 50%
Use Ceiling or Floor Fan
– Better air circulation and evaporative cooling
Consider a Mattress Cooling Pad
– Advanced pads with active cooling elements to pull heat away from the body
Making some of these bedroom environment and bedding adjustments can help those prone to getting hot during the night sleep more comfortably. Be sure to also speak with a doctor if excessive night sweats begin interfering with daily activities.
The Bottom Line
To summarize, we do experience an increase in core body temperature during sleep, particularly associated with REM sleep. The temporary spikes likely result from a combination of increased brain activity, changes in heat generation and loss mechanisms, hormonal fluctuations, and circadian rhythms. If hot flashes during sleep become bothersome, simple adjustments like lowering room temperature, using breathable bedding, and staying hydrated during the day can help sleep be more restful.
References
- Münch, M., Knoblauch, V., Blatter, K., Schröder, C., Schnitzler, C., Kräuchi, K., … & Cajochen, C. (2005). The frontal predominance in human EEG theta activity after sleep loss decreases with age. European journal of neuroscience, 21(5), 1404-1410.
- Glotzbach, S. F., & Heller, H. C. (1976). Temperature regulation and sleep-wake rhythms before and after suprachiasmatic nuclei lesions in the rat. Brain research, 103(3), 521-529.
- Obál, F., Fang, J., Payne, L., & Krueger, J. M. (1999). GHRH and sleep. Sleep medicine reviews, 3(4), 289-299.
- Kräuchi, K. (2007). The thermophysiological cascade leading to sleep initiation in relation to phase of entrainment. Sleep medicine reviews, 11(6), 439-451.