How Smart Devices Know Your Sleep Stages?

We spend about one-third of our lives sleeping, yet most people focus only on tracking their health while awake, overlooking the fact that sleep quality determines performance for the remaining two-thirds of the time. Sleep is not just a period of rest; it is a crucial stage for energy regeneration, ensuring optimal function of both the body and the brain.

One of the key factors in evaluating sleep quality is Sleep Stages—the different phases of sleep. Monitoring Sleep Stages helps detect issues such as insomnia, sleep apnea, and lack of deep sleep, while also providing valuable data for AI-powered personalized recommendations on sleep, exercise, and nutrition.

However, Sleep Stages cannot be measured directly; they must be calculated based on multiple biometric indicators. This article will explore how AI algorithms in NOTT Smartbed analyze collected biometric data to determine Sleep Stages, enabling smarter health insights.

I. What Happens During a Sleep Cycle?

Did you know that sleep isn’t just a single state but a carefully orchestrated cycle that determines how refreshed, energized, and sharp you feel every day? Your body and mind go through distinct sleep stages, each playing a crucial role in physical restoration, memory consolidation, and emotional balance.

Sleep is divided into two main types: Non-Rapid Eye Movement (NREM) sleep and Rapid Eye Movement (REM) sleep. Sleep occurs in cycles, with each cycle lasting approximately 90-110 minutes. A full night's sleep typically consists of 4-6 cycles.

Non-REM (NREM) Sleep

• Stage 1 (NREM 1 – Drifting Off): This brief stage, lasting 1-7 minutes and making up about 5% of total sleep, is when your body relaxes while remaining somewhat aware of your surroundings. Heart rate slows, breathing steadies, and brain activity begins to decrease.

• Stage 2 (NREM 2 – Light Sleep): This is where your body fully settles in, making up 45-55% of your sleep. Your temperature drops, and your brain begins processing and organizing memories. Special brain waves called sleep spindles strengthen learning and problem-solving, while K-complexes act like noise-canceling headphones, shielding you from disturbances so you can sleep soundly.

• Stage 3 (NREM 3 – Deep Sleep): Often called Slow Wave Sleep (SWS), this phase is when your body goes into full repair mode—rebuilding muscles, restoring energy, and boosting immunity. If you’ve ever woken up feeling sluggish, it might be because you didn’t get enough of this deep, restorative sleep.

REM Sleep (Rapid Eye Movement)

Around 90 minutes into your sleep, you enter REM sleep, where your brain becomes as active as when you're awake! Your heart rate and breathing speed up, and your eyes move rapidly behind closed lids. This is where your most vivid dreams occur, and your brain processes emotions, strengthens creativity, and enhances problem-solving skills.

To prevent you from physically acting out dreams, your body enters a state of temporary muscle paralysis (muscle atonia)—a fascinating built-in safety mechanism! Without enough REM sleep, you may feel emotionally off-balance, forgetful, and mentally foggy the next day.

II. How Sleep Stages Are Measured

Ever wondered how sleep trackers determine your sleep stages when they can't directly measure them? The answer lies in advanced algorithms that analyze multiple physiological signals to estimate whether you’re in light sleep, deep sleep, or REM. The more accurate and diverse the input data, the more reliable the sleep analysis.

Most sleep-tracking devices rely on three main data sources: Vital Signs, Body Movement, and EEG (Brain Waves).

1. Vital Signs: The Most Common Approach

Modern wearables—like smartwatches and smart rings—often use heart rate (HR), respiratory rate (RR), blood pressure (BP), body temperature, and blood oxygen levels (SpO₂) to estimate sleep stages. Since these vital signs fluctuate throughout the night, they provide useful clues about which phase of sleep you’re in:

However, this method has limitations:

• External factors can distort results. Caffeine, exercise, and stress all impact heart rate and heart rate variability (HRV), making it harder to differentiate sleep stages accurately.

• Sleep stages can overlap. REM sleep and light sleep (NREM 1) both involve higher heart rates and minimal movement, leading to potential misclassification.

• Wearable accuracy depends on fit. If a device is too loose or too tight, it may misread heart rate data, reducing reliability.

• It can’t detect sleep disorders. Conditions like insomnia or sleep apnea involve brain activity changes that vital sign tracking alone can’t capture. Without EEG data, wearables rely heavily on prediction models, increasing the risk of misinterpretation.

2. Body Movement

Your body moves differently in each sleep stage, and sleep trackers use this data—along with vital signs—to refine their accuracy. By analyzing how much you move and when, wearables attempt to map your sleep cycle throughout the night.

Stage 1 (NREM 1)
At this early stage, your body is still somewhat active—you may shift positions, stretch, or experience sudden twitches (Hypnic Jerks) as your brain transitions into sleep. Your eyelids grow heavy, eye movements slow down, and muscles remain active, making you easily woken up.

Stage 2 (NREM 2)
As sleep deepens, movement slows down. You may occasionally shift or readjust, but overall, your body becomes more relaxed. Muscle tone decreases, yet your body still reacts to external stimuli, preparing for deep sleep.

Stage 3 (NREM 3)
This is when your body reaches its most restful state—almost completely still, aside from slow, involuntary movements. This is also the stage where sleepwalking and sleep talking can occur. With muscles fully relaxed and brain activity at its lowest, waking up becomes difficult. Deep sleep is crucial for physical recovery and memory consolidation.

REM Sleep

During REM sleep, your body enters a state of temporary paralysis (muscle atonia), meaning you hardly move at all. However, your eyes dart rapidly (Rapid Eye Movement), breathing becomes irregular, and you may show tiny twitches in your fingers, toes, or facial muscles.

For individuals with REM Sleep Behavior Disorder (RBD), the body fails to fully paralyze, leading to uncontrollable movements—sometimes acting out dreams in ways that can be dangerous.

Limitations of Using Body Movement to Track Sleep Stages

While many wearables rely on motion tracking, this method has drawbacks:

• Inconsistent positioning: If a device isn’t worn properly or you sleep in different positions, it may misinterpret movement.

• REM sleep confusion: If you move lightly during REM, a tracker might mistake it for light sleep (NREM 1).

• Misses key sleep disorders: Sleepwalking and REM behavior disorders require EEG data for proper diagnosis—motion tracking alone isn’t enough.

3. EEG – The Gold Standard for Determining Sleep Stages

EEG (Electroencephalography) is the most accurate method for identifying sleep stages. Each sleep stage has distinct brain wave patterns that reflect neural activity throughout the sleep cycle.

• Stage 1 (NREM 1): Alpha waves (8-12 Hz) gradually decrease, and Theta waves (4-8 Hz) start to appear, marking the transition from wakefulness to sleep.

• Stage 2 (NREM 2): Theta waves dominate brain activity. Two distinct patterns emerge in this phase: Sleep Spindles (12-16 Hz), which play a crucial role in memory consolidation, and K-Complexes (0.5-2 Hz), which help prevent sudden awakenings by suppressing external stimuli.

• Stage 3 (NREM 3): Also known as Slow-Wave Sleep (SWS), this stage is characterized by Delta waves (0.5-4 Hz).

• REM Sleep: A mix of Theta (4-8 Hz) and Beta waves (12-30 Hz) results in heightened brain activity, similar to wakefulness.

Limitations of EEG in Wearable Devices

While EEG is the most precise method for sleep stage detection, few wearable devices can effectively measure EEG due to:

• Weak EEG Signals: EEG requires close contact with the scalp for accurate signal detection. There are two main types of EEG electrodes:

  • Wet EEG: The gold standard in labs and clinical settings (e.g., polysomnography, brain research). Users need to apply conductive gel and attach metal electrodes for each session. This method is highly accurate but impractical for personal sleep tracking.

  • Dry EEG: Uses specialized conductive electrodes that do not require gel, making it more suitable for at-home sleep tracking devices like smart headbands, EEG caps, and sleep monitors. However, dry EEG wearables must be worn on the head throughout the night, which can be uncomfortable and may loosen due to movement during sleep.

• Limited Multi-Functionality: Dedicated EEG devices often cannot measure vital signs such as heart rate or respiration, requiring users to purchase additional wearable devices for a complete sleep analysis.

Despite being the gold standard, EEG is not widely implemented in wearables, as these devices are primarily designed for daytime activities.

To accurately determine sleep stages, relying solely on vital signs or body movement can lead to misinterpretation. The most effective system combines all three methods: vital signs, body movement, and EEG – the gold standard for brain wave measurement. This combination minimizes inaccuracies and provides the most precise sleep data, which is exactly what the NOTT Smartbed delivers for you.

III. How Does SmartBed Know Where You Are in Your Dreams?

SmartBed tracks your sleep by combining vital signs (body temperature, heart rate, and respiratory rate) with body movement analysis. Most importantly, it features an integrated EEG sensor within the Smart Pillow, utilizing dry electrode EEG technology. This allows for precise brainwave monitoring without the need to wear any headgear—no straps, no intrusive sensors—just you and deep, uninterrupted rest.

The pillow’s sensors capture electrical signals generated by neural activity in the brain. Every stage of sleep—from the moment you doze off to the depths of your most vivid dreams—is reconstructed by SmartBed into a personalized hypnogram, a brainwave map designed just for you.

This process is completely safe, non-invasive, and comfortable. The device only reads brain signals and does not emit any electrical currents into the body. Beyond sleep staging, EEG data can also help detect irregular brain activity, providing valuable insights for medical professionals in diagnosing and treating sleep-related conditions.

Conclusion

Identifying sleep stages is a complex process that requires the integration of multiple factors. Relying solely on indicators like heart rate or body movement can lead to inaccurate results.

SmartBed sets itself apart by incorporating all three key methodologies: vital sign tracking, body movement analysis, and most importantly, EEG—the gold standard in brainwave measurement. This combination enables SmartBed to deliver more precise sleep stage detection, providing reliable insights to help you better understand your sleep quality and optimize your overall well-being.

If sleep accounts for nearly one-third of your life, why not make the most of it? Leave your email to discover how SmartBed’s cutting-edge technology can transform your sleep experience and enhance your life!