A specially-designed time-varying magnetic field was developed to treat insomnia. Clinical results showed that this method could shorten the time to go to sleep and prolong the sleep duration. However, the mechanism of this method is still not well understood. In this study, the effect of magnetic stimulation on the release of serotonin (5-HT), noradrenaline (NE), dopamine (DA) in raphe nuclei of rats, which are known to play an important role in the sleep-wake regulation, was investigated. It was shown that there was a significant difference in the release of serotonin between control group and experimental group (p<0.01). The release of serotonin of the experimental group increased significantly. No obvious release changes of NE and DA are found (p>0.05). The results indicates that one possible mechanism of inducing sleep using specially designed magnetic field is to change the release of sleep-related neurotransmitters.