What if your pajamas could tell you how well you slept? That's the dream of startup Nyx Devices, which has developed a nightshirt embedded with fabric electronics to monitor the wearer's breathing patterns. A small chip worn in a pocket of the shirt processes that data to determine the phase of sleep, such as REM sleep (when we dream), light sleep, or deep sleep.

"It has no adhesive and doesn't need any special setup to wear," says Matt Bianchi, a sleep neurologist at Massachusetts General Hospital and co-inventor of the shirt with Carson Darling, Pablo Bello, and Thomas Lipoma. "It's very easy—you just slip it on at night," says Bianchi, who has no formal role with Nyx Devices.

When people with sleep disorders spend the night in a sleep lab, they are hooked up to a complex array of sensors that monitor brain activity, muscle activity, eye movement, and heart and breathing rate. Nyx's Somnus shirt dramatically simplifies this by focusing only on respiration. "It turns out that you can tell if someone is awake or asleep and which stage of sleep they are in purely based on breathing pattern," says Bianchi. "That's a much easier signal to analyze than electrical activity from the brain."

During REM sleep, the respiratory pattern is irregular, with differences in the size of breaths and the spacing between them. Breathing during deep sleep follows an ordered pattern, "like a sine wave," says Bianchi. "And the breath-to-breath differences are very small." The lighter stages of non-REM sleep fall somewhere in between. "The motivation behind the shirt is to allow repeated measurements over time in the home," he adds. Users can log their habits, such as coffee or alcohol intake, exercise, or stress, and look for patterns in how those variables affect their quality of sleep.

Analyzing sleep stages based on respiration is still considered experimental. But Bianchi is now testing the device on patients who come to his sleep clinic who are also assessed using standard technology, known as polysomnography. The team will soon begin home tests of the shirts to further validate its use outside of the lab. The company hopes to have a commercial product available by summer of 2012 for less than $100.

The shirt is part of a growing number of devices that people can use to monitor sleep at home. The simplest, including an iPhone app, use accelerometers to measure movement, giving a rough gauge of when people fall asleep and wake up. A more sophisticated consumer device that monitors electrical activity from the brain and muscles, called the Zeo, came on the market two years ago.

While Nyx envisions the shirt as a consumer product, Bianchi wants to use it for his patients. Bianchi's previous research has shown that people with insomnia often underestimate how much they sleep, so he wants to determine whether giving them an objective way to measure sleep will help them reassess their condition and improve quality of sleep. "It will be a game changer for my clinical practice," he says. "There are zero objective tools available to physicians to assess insomnia."