What Is Photoplethysmography (PPG)? How 100 Hz Near-Ear Sensing Reads Your Nervous System

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Questions

What is photoplethysmography (PPG) in simple terms?

PPG is a way of measuring blood flow changes using light. A sensor shines an LED into the skin and measures how much light bounces back; because blood absorbs light, the returning signal dips with every heartbeat. From that waveform, you can derive heart rate, heart-rate variability, and blood-oxygen levels.

How does a PPG sensor measure stress?

PPG does not measure stress chemically. It measures heart-rate variability, the millisecond variation between beats, which reflects the balance between your sympathetic (stress) and parasympathetic (recovery) nervous systems. Sychedelic computes RMSSD, LF/HF, and pNN50 from PPG over a rolling 120-second window to classify your cognitive state.

What is the difference between PPG and ECG?

ECG measures the heart's electrical activity through skin electrodes; PPG measures the optical signature of the resulting blood-volume pulse. ECG is the gold standard for beat timing, but PPG is far easier to wear continuously, which is why nearly all consumer wearables use it.

Is PPG accurate for HRV?

Yes, within limits. Accuracy depends heavily on sampling rate, sensor placement, and motion. Higher sampling rates give finer beat-to-beat timing resolution, which is what RMSSD depends on — at 100 Hz, timing resolves to roughly 1.6 ms, well inside the 3–8 ms differences that separate cognitive zones. A stable, low-motion measurement site improves accuracy further.

Why does PPG use green light?

Green light (~530 nm) is strongly absorbed by hemoglobin and penetrates only shallowly, which makes it robust against motion artifact and well-suited to detecting the surface pulse. Red and infrared light penetrate deeper and are used together to estimate blood-oxygen saturation (SpO₂).

Why measure at the temple instead of the wrist?

The superficial temporal artery at the temple is a terminal branch of the external carotid artery, a larger, higher-flow vessel than the wrist's radial artery. It also moves far less during seated desk work, which protects the beat-to-beat timing that HRV depends on.

What sampling rate does Sychedelic's PPG use, and why?

100 Hz, roughly four times the ~25 Hz of many budget wrist sensors. That resolves beat timing to within about 1.6 ms, fine enough to keep RMSSD reliable inside the short 120-second window used for real-time cognitive-zone classification, where the differences between zones can be as small as 3–8 ms.

What can PPG not tell me?

PPG cannot read brainwaves, measure cortisol, or detect "stress" as a biochemical state. It captures the cardiovascular fingerprint of autonomic activity, a reliable proxy, not a direct measurement of brain or hormone activity.

What is RMSSD, and why does it matter?

RMSSD is the root mean square of successive differences between heartbeats and is the primary time-domain measure of parasympathetic (vagal) activity. Higher RMSSD generally indicates better recovery and readiness, which is why Sychedelic uses it as a core input to its session gate.

How does PPG fit into a closed-loop device?

In a closed-loop wearable, the sensor's output controls the intervention. Sychedelic's PPG reads HRV continuously, an algorithm classifies your zone, and tDCS only activates when you reach the Focused zone and electrode contact passes an impedance check.

Does hair or movement affect the reading?

Yes, hair at the temple can interrupt the optical path, and large head movements add noise. Sychedelic mitigates this with a stable near-ear housing, a live signal-quality indicator in the app, and a 120-second averaging window that smooths over brief interruptions.

Is PPG the same technology as a pulse oximeter?

Effectively, yes. A fingertip pulse oximeter is a two-wavelength PPG device: it compares red and infrared light absorption to estimate blood-oxygen saturation, on top of the same pulse signal used for heart rate.