Sleep Monitoring with a Fitness Tracker

What is Sleep Tracking?

Sleep is more than just the simple act of “resting the eyes”. Some of us feel well-rested after only a couple of hours of sleep while some of us need more than 8 hours. Some of us barely dream while some of us often have nightmares. Regardless of which group you fall in, your body still performs the same essential functions each night during sleep.

Sleep tracking has been used for decades to diagnose neurological and psychological disorders, but with the increased availability of wearables, it has been trending among the general public.

Sleep tracking looks at multiple components that affect sleep:

  • how long you sleep

  • when you wake throughout the night and in the morning

  • different phases of the sleep cycle you go through

  • your respiratory and heart rate during the night

  • body temperature while sleeping

Some trackers even consider lifestyle and environmental factors, such as caffeine intake or the temperature in your room. Collecting data from these areas can help guide how you can improve your sleep.

Sleep Stages

There are four stages of sleep in one sleep cycle and people typically go through at least one to two cycles in an 8 hour period of sleep.

Light Sleep

Light sleep includes stages 1 (N1) and 2 (N2) of the sleep cycle. N1 starts right when you fall asleep and typically lasts around 1-7 minutes. It’s the easiest stage to wake up from. N2 follows next and lasts around 10-25 minutes. It starts to become more difficult to wake up from this stage. You’ll find that your heart and breathing rate will slow and your body temperature will drop slightly at these stages. Your muscles will start to relax in the N2 stage.

Deep Sleep

Deep sleep is stage 3 (N3) of the sleep cycle. This stage is also called delta sleep or slow wave sleep, as brain waves have a distinct delta wave pattern. This stage typically lasts 20-40 minutes but often shortens as you move through multiple sleep cycles.

It’s the most difficult to wake up from this stage. Your body and muscles become even more relaxed, while your breathing and heart rate slow to their lowest of the night. This might be the stage where your sleep tracker records the slowest heart rate throughout your sleep. This stage is a very important contributor to your body’s recovery and can help boost your immune system. You may have woken up from a deep sleep stage on mornings when you feel groggy waking up and starting your day.

REM Sleep

Though dreams can occur in other sleep stages, REM or rapid eye movement sleep is most commonly associated with dreaming. As per the name, your eyes will move rapidly under your eyelids as you sleep. You might also have heard stories of people having sleep paralysis, which can occur during the REM stage. In this stage, your muscles are paralyzed and if you wake up from this stage, your body might still be in REM mode and thus you are unable to move your muscles. This stage can last from a couple of minutes to an hour and will increase in length as you go through the sleep cycle. You’ll typically spend the most time in REM sleep and it is very important for brain and memory function in particular.

How does a wearable track sleep?

Wearables typically use gyroscopes and accelerometers to track movement throughout the day and while you sleep. This data is used to estimate when you’re asleep. This process is known as actigraphy. While this is helpful for sleep duration, it is not as predictive of sleep stages. For estimating sleep stages, wearables use photoplethysmography (PPG). PPG involves light sources and photodetectors that measure how the blood absorbs and reflects the different light wavelengths that tell us about blood volume, oxygen saturation, and heart rate. This data is used to then predict stages of sleep. For example, the data can predict that a person is in stage 1 when oxygen levels and heart rate starts to drop.

How accurate are sleep trackers?

The accuracy of sleep trackers is unclear and inconsistent because wearables rely on indirect measures of sleep. Many studies have found wearables that use actigraphy to overestimate total sleep time and underestimate episodes of wakefulness after sleep occurs. Studies have shown that PPG may be more useful in predicting sleep duration but not as much with sleep stages. Accuracy can vary greatly among devices and some more advanced wearables are more accurate. Polysomnography remains the gold standard for measuring sleep, but relying solely on a wearable may not accurately help diagnose a disorder. However, they are certainly helpful in providing insights into sleep hygiene and tracking sleep changes or improvements over time.

You can find the best fitness trackers for sleep by selecting “Track Sleep” as your primary purpose in our fitness tracker quiz or check out the products page to view all fitness trackers.

This is for informational purposes only and should not be considered as medical advice. Go to the emergency room if you are experiencing a life-threatening medical emergency. Disclaimer: Takeoff Health may make a small commission from some of the links above.

Blog is written by Ann Pongsakul, a resident physician in family medicine.

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