A Dream Solution for Sleep Disorders

If you are like most hardware hackers, your hobby projects probably keep you up into the early hours of the morning more often than you’d care to admit. So when your head finally does hit the pillow, you need to get some quality shut-eye, pronto. After all, your coffee-making PC and your text messaging gun are not going to pay the bills, so you need to be feeling fresh and ready to go for work the next morning.

But if you cannot seem to get circuit diagrams and source code out of your head after you hit the sack, you could find yourself with a real problem. Before a sleep disorder of this sort can be treated, the root problem needs to be diagnosed. This is traditionally done with a sleep study in a clinical setting. But if you are already having trouble sleeping, there is no way you are going to sleep in the doctor’s office with wires stuck to your body.

A more convenient and comfortable diagnostic device may be on the horizon, however. Researchers at Columbia University and the Hong Kong University of Science and Technology have developed a sensing system that is built entirely into a pillowcase. Called PillowSense, their creation can monitor certain physiological parameters and factors like sleep position and movement that are essential in the diagnostic process.

PillowSense contains a dual-layer fabric sensor array. The top layer handles the electrical side of things with conductive fabric electrodes that measure biopotentials like electrocardiogram (ECG) and surface electromyography (sEMG) signals from the sleeper’s neck. Beneath this lies a pressure-sensitive layer made from piezoresistive films sandwiched between conductive fabric sheets.

The electrodes themselves are small patches of conductive fabric mounted on concave bases built from kinesiology tape, which improves skin contact without causing discomfort. Each patch provides six signal channels (vertical, horizontal, and diagonal) so that even if the pillow shifts, at least some channels line up with the body’s natural electrical activity. ECG is strongest when the signal path matches the heart’s axis, while sEMG benefits from channels that avoid ECG interference.

Conductive thread and flexible printed circuit boards round out the design. The pressure units and biopotential patches are paired and wired into a control board at the pillow’s edge, where the bulk of the electronics sit out of the way. Processing duties are split between an ESP32-C3 microcontroller, which handles pressure sensing and electrode selection, and an OpenBCI Cyton board, which digitizes the bioelectric signals at 250 Hz. Data is transmitted via Bluetooth Low Energy to a computer for further processing by machine learning models.

In a study spanning 151 nights and over 900 hours of sleep, PillowSense reached an F1 score of 88% for classifying seven different sleep postures, and its heart and respiratory rate measurements matched up closely with medical-grade equipment. The system performed well even when more than one person was sharing the bed.

The whole pillowcase can be detached, sanitized, and even washed without ruining the sensors. Study participants also reported that it was comfortable to use, so PillowSense looks like a practical and viable option for real-world use. Sleep studies might be a whole lot less burdensome in the near future.