by Dr Mercola 

If you’ve ever woken up because you’re too hot or cold, you know that temperature extremes can significantly affect your sleep quality. But even subtle differences in temperature can influence your sleep, for better or worse. Your body temperature cycles along with your sleep-wake rhythm, decreasing at night while you’re asleep and increasing during the day.

You’re most likely to sleep when your core body temperature decreases and unlikely to sleep while it’s on the rise. Further, after you fall asleep, your core body temperature will decrease even more, but your peripheral skin temperature, which plays a role in maintaining your body’s core temperature by adjusting blood flow to your skin, remains high.

Both your core body temperature and your peripheral skin temperature are further influenced by external factors in your sleep environment, such as whether you sleep nude or in pajamas, and whether you use a comforter while you sleep.

Taken together, if you’re having trouble sleeping, paying attention to your thermal environment and making changes where necessary could help you get a good night’s rest.

How Heat Exposure Affects Your Sleep

In people who slept nude except for shorts, without any covers and, for research purposes, on a bed made from nylon webbing, the ambient temperature of 21 degrees C (69.8 degrees F), which was the coolest temperature tested, was most disruptive to sleep.

Cold ambient temperatures were generally more disruptive to the participants’ sleep than warm temperatures, although sensitivity to heat or cold varied according to the person. Further, this doesn’t take into account the way many people sleep, which is wearing pajamas and under the covers.

“In real-life situations where bed covers and clothing are used, sleep is actually disturbed during heat exposure rather than cold exposure in the young, as well as in the elderly,” researchers wrote in the Journal of Physiological Anthropology.

Heat combined with high levels of humidity may be worst of all, as humid heat exposure suppresses the decrease in core body temperature that normally occurs with sleep, leading to increases in wakefulness and decreases in rapid eye movement (REM) and slow wave sleep (SWS) stages. The researchers continued:

“Humid heat exposure most probably increases heat stress because of the difference in the sweat response caused by the humidity.

Decreased ambient humidity allows sweat to evaporate, thereby dissipating the heat, whereas increased humidity does not allow the sweat to evaporate, causing the skin to remain wet. The dripping sweat and increased skin wetness decrease the sweat response due to hidromeiosis preventing dehydration.”

Also important, exposure to humid heat appears to have a more significant effect on your sleep when it occurs in the initial segment of sleep as opposed to later. If you’re only going to use air conditioning for a few hours a night, then it makes sense to turn it on when you’re first going to sleep.

Exposure to higher temperatures at night may be particularly disruptive for elderly men, a population easily awakened by even mild heat exposures and who have an already reduced heat tolerance.

What Does Cold Exposure Do to Your Sleep?

Whereas heat exposure has a greater effect on the first segment of sleep, cold exposure primarily affects the later segment, when REM sleep occurs. In people sleeping semi-nude, the disruption occurs due to the suppression of your body’s thermoregulatory response. However, cold is unlikely to have as great an effect in real-life situations, when most people use sleepwear or bedding to offset cold temperatures at night.

According to the Journal of Physiological Anthropology study, in real-life settings using clothing and/or bedding, no significant differences were observed in sleep at temperatures ranging from 13 degrees C (55.4 degrees F) to 23 degrees C (73.4 degrees F).

That being said, although cold exposure doesn’t appear to affect sleep stages, it did lead to changes in autonomic heart activity, enough so that researchers suggested it could be one reason why cardiac events peak during cold winter temperatures. In fact, research suggests that mortality from ischaemic heart disease is associated with cooler homes and limited bedroom heating.

If you take this into account, the Journal of Physiological Anthropology researchers noted, “… the impact of cold exposure may be greater than that of heat exposure in real-life situations; thus, further studies are warranted that consider the effect of cold exposure on sleep and other physiological parameters.”

Increases in Nighttime Temperatures Linked to Worse Sleep

When measured subjectively, data from 765,000 Americans showed that increases in nighttime temperatures increased self-reported nights of insufficient sleep. Other research also found that high temperatures affected multiple aspects of sleep, leading to:

  • Worse sleep duration 
  • Shallow sleep 
  • Less sleep calmness 
  • Difficulty falling asleep 
  • Lower sleep satisfaction

Sleeping in a cooler room may therefore lead to fewer disruptions in your sleep, with the National Sleep Foundation (NSF) suggesting the ideal bedroom temperature is somewhere between 15.5 degrees C (60 degrees F) and 19.4 degrees C (67 degrees F).

You want to avoid extreme temperatures (either too hot or too cold), as these could activate thermoregulatory defense mechanisms that cause you to wake up. NSF noted:

“If your room is cool, rather than warm, it will be much easier to shut your eyes for the night. Thermostat settings far lower or higher than what’s recommended could lead to restlessness and can also affect the quality of [REM], the stage in which you dream. It can also help to think of your bedroom as a cave – it should be quiet, cool, and dark for the best chance at getting enough rest.”

Showering Before Bed and Wearing Socks to Bed May Improve Your Sleep

Raising your skin temperature promotes the onset of sleep, as it increases neuronal activity in brain areas involved in sleep regulation.

An increase in skin temperature of just 0.4 degrees C has been found to suppress nighttime wakefulness and shift sleep to deeper stages in the young, the elderly and people with insomnia – even though it didn’t alter core body temperature.

The effect was especially pronounced in the elderly, with researchers writing in the journal Brain:

“Elderly subjects showed such a pronounced sensitivity, that the induced 0.4 degrees C increase in skin temperature was sufficient to almost double the proportion of nocturnal slow wave sleep and to decrease the probability of early morning awakening from 0.58 to 0.04.

Therefore, skin warming strongly improved the two most typical age-related sleep problems; a decreased slow wave sleep and an increased risk of early morning awakening.”

There are a number of ways to take advantage of this science in your bedroom. One simple method is to take a shower before bed. A 10-minute shower at a temperature of about 40 degrees C (104 degrees F) increased skin temperature and helped participants fall asleep faster and maintain greater sleep efficiency.

Soaking your feet in hot water before bed may also help and has been shown to help people fall asleep faster. Wearing socks to bed is another option. When participants in one small study wore socks to sleep, they fell asleep faster, slept longer and had fewer nighttime awakenings.

An interesting aside – people with insomnia who wore special caps filled with cool circulating water were able to sleep better, in this case likely because the cap helped to cool their brain. Many insomniacs report that they can’t fall asleep because they ‘can’t turn their brains off’ at night. The extra brain activity was keeping their brains too hot for sleep, and the cap helped resolve this.

Sleeping in a Cool Room Increases Beneficial Brown Fat

Another reason to sleep in a cool room is its beneficial effect on brown fat, which generates heat by burning calories in order to help maintain your core temperature. Sleeping in a cool room (19 degrees C or 66.2 degrees F) for four weeks doubled the volume of brown fat in study volunteers, improving insulin sensitivity at the same time.

Shivering may be the mechanism that triggers brown fat to produce heat and burn calories, but if you’re shivering it’s not conducive to sleep. Finding the sleep temperature Goldilocks zone is what you’re after – cool enough to help you sleep and increase brown fat but not so cool that it makes you uncomfortable.

The exact best temperature will vary by individual, but sleeping in a cool room with a thin sheet and blanket is generally enough to keep your skin temperature warm, so you feel comfortable, while still benefiting from the cool sleep temperatures.

Proper Sleep Is Essential for Good Health

Adults need an average of seven to nine hours of sleep a night, with most doing well with about eight. If you have trouble achieving this duration, or you wake frequently during the night, it’s time to take steps to improve your sleep hygiene, starting with your bedroom – but temperature is only one factor.

Be sure you’re sleeping in complete darkness, as light (even from a night light or alarm clock) can disrupt your internal clock and your production of melatonin and serotonin, thereby interfering with your sleep.

In the morning, bright, blue light-rich sunlight signals to your body that it’s time to wake up. At night, as the sun sets, darkness should signal to your body that it’s time to sleep. Ideally, eliminate electromagnetic fields (EMFs) and shut down the electricity to your bedroom by pulling your circuit breaker before bed and turning off your Wi-Fi at night.

This is just a starting point. Other ways to improve your sleep including adopting a neutral sleeping position, going to bed earlier and considering a separate bedroom if your partner is interfering with your sleep.

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