Every moment of the day and night, your body is working hard to maintain an ideal internal temperature. It’s a remarkable ability given the various environments and activities we put our bodies through. We are frequently in and out of hot and cold places with different layers of clothing. We can jump into a freezing lake or poach ourselves in a sauna and our ever-vigilant nervous system will adjust our internal thermostat. 

Because heat regulation (aka thermal homeostasis) is so critical to your daily health, Fitbit has introduced a new sensor on Fitbit Sense to track your skin temperature as you sleep.¹ A dedicated tile on your Fitbit Today dashboard will help you track your skin temperature variation from the previous night’s sleep or see when your average skin temperature variation is higher or lower than your baseline. The tile will also allow you to manually log your core temperature. The temperature tracker is one of several new features Fitbit has launched to help users better understand their bodies and achieve well-being. 

While the average human core temperature is 98.6 degrees, it is normal for the body to fluctuate a couple of degrees. A healthy body’s internal temperature will change by about 2 degrees in sync with your 24 circadian rhythms. It is normal to run a bit cooler when we go to bed and warm up when we become active. Changes in hormones over a monthly menstrual cycle, as well as exercise and eating habits, can also change your internal temperature. 

The largest organ in your body—your skin—takes a central role in regulating heat. When your nervous system senses that our internal temperature is rising, the blood capillaries near the skin dilate to take advantage of the sweat evaporating off the skin surface. When body temperature drops, our nervous system restricts blood flow near the skin and moves it into the body’s more insulated core. 

Because the skin is more exposed to the environment, skin temperature fluctuates more widely than core temperature. Skin will cool due to sweat and evaporation, for instance, as the body works to bring down your internal temperature. For this reason, skin temperature shouldn’t be used as a substitute for a thermometer that measures core temperature. It can, however, function as a warning sign that something has changed. 

Once you’ve established your nightly baseline, it is easy to see when your data varies from your baseline. These changes can be an important signal of disequilibrium, including the onset of fever due to illness. 

Temperature can reveal other insights as well. During a normal circadian cycle, the body’s core temperature drops at bedtime and bottoms out in the early morning hours. These core temperature patterns can be reflected in skin temperature changes. Peripheral skin temperature rises in the evening in order to help release heat from the body’s core. That cooling-off is part of what helps you go to sleep. 

Both eating and exercise can increase the body’s temperature and make it more challenging to get to sleep. Unusual spikes or drops in your nighttime skin temperature may also prompt users to experiment with different room temperatures or types of sheets and blankets. 

“I think temperature comfort during sleep is important and something that individuals might learn about through the skin temperature sensors,” said Lindsey Sunden, Director of Physiological Sensing at Fitbit. “If you see peaks in your skin temperature during the night, it could be a sign that overheating is disrupting your sleep.” 

Currently, the skin temperature sensor is not intended to diagnose or treat any medical condition. Instead, it is part of a suite of new metrics that include oxygen saturation, breathing rate, and heart rate variability sensors designed, in concert, to give users new insights into their well-being. 

“Through our millions of users we are collecting data on billions of nights of sleep,” said Sarah Solso Abruscato, a Product Marketing Manager at Fitbit. “Understanding what that data reveals will be critical to our mission, which has always been to make the world healthier.”