Table 2.

Advantages and disadvantages of wearable sleep devices that measure each of the five popular signal types and their recommended design requirements for successful sleep measurement.

Signal type	Advantages	Disadvantages	Design requirement
EEG	•Highest accuracy in specific sleep stage analysis and detection of various sleep disorders.	•The device should be placed on the head; it may bother the user's sleep. •Requires direct contact between sensor and skin. •Signal quality depends on skin condition.	•Reasonable inter-electrode distance to ensure good signal strength. •High-resolution DAQ system to measure brain waves. •Combination with EOG and chin EMG for higher accuracy •Novel material for the electrode to ensure reliable contact with skin. •Minimized sleep disturbance with the novel form factor.
ECG	•Gold standard in cardiac monitoring and cardiac disorder diagnosis.	•Sensor location is usually restricted to chest to obtain high SNR. •Requires direct contact between sensor and skin. •Signal quality depends on skin condition.	•Reasonable inter-electrode distance to ensure good signal strength. •Novel material for electrode to ensure reliable contact with skin. •Minimized sleep disturbance with novel form factor. •Placement of sensor on chest to ensure high signal quality with minimized motion artifact.
PPG/SpO2	•Smaller, cheaper replacement for ECG. •Can be measured from multiple different body locations (wrist, nose, etc.). •SpO2 is effective in diagnosis of apnea.	•Although it can be measured on various body parts, just few locations (finger, earlobe, etc.) provide reliable signal quality, especially for SpO2. •Requires direct contact between sensor and skin.	•Delicate control of number, configuration, wavelength, and brightness of LED and photodiode. •Choice of measurement location to maximize signal strength. •Wearable platform that can apply moderate pressure on skin (e.g., wristband).
Respiration (chest expansion)	•Most effective in diagnosis of sleep apnea events characterized by frequent stop in breathing.	•Provides little information for sleep analysis besides apnea events.	•Novel sensor material to maximize sensitivity (e.g., TENG). •Minimized sleep disturbance with novel form factor.
Actigraphy (body movement)	•Small sensor size. •Does not require direct contact between sensor and skin. •Comfortably worn in a wristband-type device.	•Despite reasonable accuracy in wake/sleep detection, specific sleep staging suffers lower accuracy.	•Combination with PPG and ambient light sensor to improve accuracy in sleep/wake detection.