One major problem with antidepressants is that patients have to take the drug for at least a month to know whether or not it works. But that could soon change after researchers found a quicker, more effective way to gauge the effectiveness of an antidepressant: monitoring a patient’s brainwaves as they sleep.

As reported in New Atlas, a team of scientists from Switzerland’s University of Basel ran a small pilot study that recruited 37 test subjects, all of who were already being treated for major depression. During the study, they all regularly had their brain waves measured during the REM (rapid eye movement) phase of sleep. A control group of 15 of the volunteers simply stayed on their existing medication for the duration of the study. The other 22, though, were regularly switched to different medications if their brain wave activity suggested that the current drug was likely to be ineffective.

Albeit a small study, the results were impressive. After five weeks, 87.5 percent of the people in the second group experienced a significant improvement in their condition, as measured by the standard Hamilton Depression Rating Scale. By contrast, only 20 percent of the control group experienced a similar response.

Moving forward, the researchers plan to conduct a larger-scale study to further test the effectiveness of monitoring brainwaves during sleep to test antidepressants.