Abstract: Prior observations of low physical activity in psychiatric inpatient care suggest a risk of physical deconditioning, but to date no studies have explicitly investigated this possibility by measuring change in cardiovascular fitness over the course of hospitalization. The present study used mattress actigraphy to obtain a passive measure of sleep heart rate (sHR) among a sample of 111 male Veterans receiving treatment for PTSD at the VA Palo Alto Health Care System Trauma Recovery Program (TRP). A linear mixed-effect regression model indicated that sHR significantly increased over nights hospitalized, though this effect was attenuated among those who participated at least once in a voluntary cycling program. Conversely, higher BMI at intake was associated with greater increases in sHR over nights. These findings provide evidence of physical deconditioning in the context of residential psychiatric treatment, while suggesting that at least some patients are protected from its impacts. Whole health interventions that promote exercise in tandem with mental health treatment may help to counteract physical deconditioning in psychiatric inpatient settings and should be designed to support patients of diverse fitness levels.