Written by: Lana Adler
Updated November 20, 2020
A slight dip in your body temperature while you sleep is part of the normal sleep cycle for humans and other mammals. These changes are the result of your natural circadian rhythm that regulates when you feel tired or awake. Your core temperature gradually decreases in the hours before bedtime, and reaches its lowest level during the initial stages of your sleep cycle -- otherwise known as non-rapid eye movement (NREM) sleep. It remains low for most of the night and returns to its normal level right before you wake up.
Your sensitivity to temperatures also changes while you're asleep, and environments that are either too hot or too cold can negatively affect the duration and quality of your sleep. For this reason, you should take measures to ensure a comfortable temperature for both your bed and bedroom.
Why Does Your Body Temperature Change During Sleep?
The circadian clock -- also known as the suprachiasmatic nucleus (SCN) -- is a cluster of thousands of cells found in the hypothalamus area of the brain. A nerve tract connects the SCN to the retinas in your eyes. Based on communications between the eyes and SCN, you follow a 24-hour cycle of sleep and wakefulness known as the circadian rhythm.
During the day, your eyes perceive natural light and transmit a signal to the SCN indicating that you should be awake. This stimulates production of cortisol, a hormone that induces feelings of alertness. Your core temperature also remains steady during the day in order to keep you energized.
As daylight fades and evening begins, the pineal gland in your brain releases melatonin (another hormone) to make you feel tired and relaxed. The period of inactivity that follows lowers your rate of metabolic heat production, which in turn causes your body temperature to gradually decrease. As a result, these changes to your core temperature coincide with feelings of sleepiness.
This loss of body heat helps you fall and remain asleep throughout the night. Most people begin shedding body heat about two hours before they go to bed and maintain a reduced body temperature while sleeping.
How Does Your Sleep Cycle Affect Body Temperature?
A normal sleep cycle in humans consists of four distinct stages characterized by body temperature shifts and other physiological changes. The first three stages are considered non-rapid eye movement (NREM) sleep, and the last stage is considered rapid eye movement (REM) sleep.
- NREM 1: This transitional stage begins as soon as you fall asleep and lasts for several minutes. NREM 1 is considered "light sleep." Your core temperature decreases slightly, along with your heartbeat and breathing rates. Eye movements and brain wave activity begin to slow down, and the muscles relax – though you may experience movements known as hypnic jerks. Additionally, your brain temperature will decrease by approximately 0.2 to 0.4 degrees.
- NREM 2: This is the second and final stage of light sleep. Your body temperature drops to its lowest point, which is typically 2 degrees Fahrenheit below your normal temperature during the day. Eye movements cease completely, heartbeat and breathing rates continue to decrease, and your muscles relax further. Your brain temperature also decreases by roughly 0.2 to 0.4 degrees. NREM 2 is normally the longest of the four sleep cycle stages.
- NREM 3: The third stage marks the beginning of slow-wave sleep, or deep sleep, during which your temperature remains low and begins to decrease a bit further. Brain wave activity, heartbeat, and breathing rate also reach their lowest levels of the sleep cycle, and muscles relax completely. NREM 3 is a longer stage during the first part of the night, and decreases in duration during the latter half of your sleep.
- REM: This fourth and final stage kicks off about 90 minutes after you fall asleep. Unlike the NREM stages, REM sleep is characterized by increased brain activity and eye movements. However, your body temperature remains lower. Most dreaming is believed to occur during REM sleep, and your body is partially paralyzed. This mechanism prevents you from physically acting out against dreams.
These four stages repeat every 90 to 120 minutes throughout the night, allowing you to experience roughly four to five different cycles before waking. Your body then returns to its baseline level by the time you wake up in the morning. This rewarming process allows you to feel energized and refreshed when you get out of bed and start your day.
How Does External Temperature Affect Sleep?
Exposure to excessively cold or excessively hot temperatures can affect how you sleep, albeit in different ways. You're more sensitive to cool or warm temperatures during NREM stages of your sleep cycle, and thus more likely to wake up if you feel too hot or too cold. Thermosensitivity is not as strong during REM sleep, but you may still wake up if dramatic temperature shifts occur.
What Is the Best Temperature for Sleep?
While everyone has different preferences, most experts agree the best temperature for sleep falls between 60 and 67 degrees Fahrenheit (15.6 to 19.4 degrees Celsius). This range ensures your core temperature won't rise excessively while you sleep, as an increase in body heat can cause you to wake up. Specifically, 65 degrees Fahrenheit (18.3 degrees Celsius) is considered ideal.
If 60-67 degrees sounds too cold for you, try sleeping under blankets or wearing bedclothes. You can always remove these layers during the night if you begin to feel too warm.
How Hot Temperatures Affect Sleep
Hot or warm temperatures can disrupt sleep by offsetting your natural decrease in body temperature. Humid nights can be particularly problematic because they can induce sweating in bed, which can cause you to wake up during the night. This negatively affects sleep efficiency, or the ratio of how long you actually sleep at night to the total amount of time you spend in bed.
Studies have found people who wear bedclothes or lie under quilts, comforters, and other thick bedding materials are most sensitive to heat exposure while they sleep. Hot environs not only stimulate wakefulness, but also decrease the amount of time you spend in NREM 3 slow-wave and REM sleep. Both of these stages play important roles in memory consolidation, cell repair, and other vital bodily processes that help you feel refreshed and rejuvenated in the morning.
Age can also be a factor in how you respond to heat while you're asleep. Young children and elderly people are more likely to experience heat-related sleep disturbances than older children and adults.
How Cold Temperatures Affect Sleep
Although your core body temperature decreases at night, studies have shown exposure to excessively cold ambient temperatures can impact your sleep quality. This is especially true for people who sleep in the nude or semi-nude, or who do not use blankets and other covers in bed. Additionally, you'll probably have a harder time falling asleep if your feet feel cold.
During a normal sleep cycle, your blood pressure will gradually decrease during the NREM 1 and NREM 2 stages before bottoming out during NREM 3. Once REM sleep begins, blood pressure elevates back to normal waking levels before the sleep cycle begins anew. Exposure to cold can cause your blood pressure to increase during NREM sleep, which may reduce the amount of time you spend in REM sleep. This is why bedding is important for people who prefer to sleep naked, as well as those who naturally run cold in bed.
Keeping warm in bed is critical during colder times of the year, especially for the elderly or anyone with cardiovascular problems.
Tips for Maintaining a Comfortable Sleep Temperature
If you tend to feel too hot or too cold in bed and your temperature sensitivity is negatively impacting how you feel in the morning, there are measures you can take to sleep more comfortably. These include:
- Set your bedroom thermostat: While 65 degrees Fahrenheit is considered the best sleep temperature, most experts agree 60 to 67 degrees is a reasonable range for most people. Even if you find cooler or warmer temperatures more comfortable while you're awake, exposure to too much heat or cold can still interfere with your sleep cycle.
- Invest in a good fan: Even if you rely on an air conditioner in your bedroom, a fan will circulate air for added cooling. If you'd rather not make this purchase, there are a few hacks you can try instead. These include keeping an ice pack or glass of ice water on your nightstand, or placing your pillowcase in the freezer before bedtime.
- Take a hot bath before bed: This may seem counterintuitive, but lying in hot or warm water (8) can actually help you sleep cooler. Not only are baths relaxing, but your body temperature will dip as soon as you leave the tub and this can promote feelings of sleepiness.
- Consider blackout curtains: Blackout curtains are designed to block outside light, which helps promote sleep. As an added bonus, these curtains also keep your bedroom cooler during hotter months and offer added insulation when the temperatures drop.
- Experiment with layers: Light bedclothes can provide good insulation against the cold without making you overheat, while sleeping naked can help your body maintain a lower temperature in hot and humid climes. If you prefer to sleep clothed, natural fibers like cotton, linen, and silk offer better breathability than synthetic fabrics such as polyester. Be sure to keep your feet warm, as well.
- Optimize your bedding: Breathable sheets are a good balance for most sleepers. They insulate fairly well during the colder months but also won't trap too much heat when the temperatures peak in the spring, summer, and early fall.
- Pick the right mattress: A cooling mattress can make a huge difference. Certain mattress materials, such as memory foam, can trap body heat at the surface and cause you to sleep hot. Innerspring and hybrid mattresses usually sleep cooler by comparison because their open coil systems promote steady airflow throughout the interior. Beds with ventilated latex layers also provide above-average temperature control.
- Don't exercise too close to bedtime: Moderate exercise during the day will expend energy and help you wind down in the evening. However, you should avoid working out in the hours leading up to bedtime. Elevating your body temperature can make it harder to fall asleep.
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