Cranial Electrotherapy Stimulation: Piloting a Road to PTSD Prevention in First Responders

The goal of this study is to test whether active duty firefighters find it possible and suitable to do cranial electrotherapy stimulation (CES) at home, and test whether CES influences measures of stress. The main questions it aims to answer are: * is CES feasible and acceptable in a population of firefighters, and * does CES changes feelings of fatigue, anxiety, and brain connectivity in firefighters. Participants will * complete four weeks of CES at home, and * complete daily assessments of fatigue and anxiety, and maybe asked to * complete an MRI scan before and after CES, and * wear a device to measure their heart rate and sleep quality.